<PAGE>   1
                                                                 EXHIBIT 2.1(b)

                  ADDENDUM NO. 2 TO STOCK PURCHASE AGREEMENT

       This Addendum (the "Addendum"), dated effective as of this 29th day of
 September 1995, by and between Landmark Graphics Corporation, a Delaware
 corporation ("Landmark"), and the former owners (the "Earnout Recipients") of
 the Class A Common Shares, no par value per share, of MGI Associates, Inc., a
 Texas corporation ("MGA"), evidences that,

       WHEREAS, Landmark and the Earnout Recipients are parties to that certain
 Stock Purchase Agreement, dated as of September 29,1994, as amended by
 Addendum No. 1 to the Stock Purchase Agreement (the "Stock Purchase
 Agreement");

      WHEREAS, Article 4 of the Stock Purchase Agreement contemplated that
Landmark would, based upon the financial performance of MGA, make payments of
money to the Earnout Recipients;

      WHEREAS, Landmark and the Earnout Recipients desire to amend the Stock
Purchase Agreement to eliminate the financial performance of MGA as a factor in
determining when and the extent to which such payments are to be made by
Landmark;

       NOW THEREFORE, in consideration of the premises and other good and
valuable consideration, receipt and sufficiency of which are hereby
acknowledged, the parties hereto agree as follows:

                                  ARTICLE I
                                   GENERAL

       SECTION 1.1  RELATION TO STOCK PURCHASE AGREEMENT.  Capitalized terms
used but not defined herein are used as defined in the Stock Purchase
Agreement.  Except as modified herein, the Stock Purchase Agreement will remain
in full force and effect in accordance with the terms thereof.  In the event of
any conflict between the terms and conditions of the Stock Purchase Agreement
and the terms and conditions of this Addendum, the terms and conditions of this
Addendum will govern and control.

       SECTION 1.2  AMENDMENT OF ARTICLES 3 AND 4.  Articles 3 and 4 of the 
Stock Purchase Agreement are hereby amended to read in their entirety as set 
forth on Exhibit 1.2 attached hereto and incorporated herein by reference.

       SECTION 1.3  CONTINUATION OF THE STOCK PURCHASE AGREEMENT.  Except as
amended by this Addendum, the Stock Purchase Agreement will remain in full
force and effect, in accordance with its terms.

       SECTION 1.4  NO DUPLICATE PAYMENTS.  Nothing contained in this Addendum
will be deemed to obligate Landmark to make more than one (1) payment with
respect to the




<PAGE>   2
Earnout Period ending June 30, 1995.  Such payment will be made on or before
September 30, 1995 and will be made in an amount equal to $1,808,095.00.

       SECTION 1.5  AUDIT OF RESULTS OF OPERATIONS.  On or before October 30,
1995, Landmark will cause the revenues of MGA and its subsidiaries for the
fiscal year ending June 30, 1995 to be audited by Arthur Andersen LLP.  Such
audit will be performed in accordance with generally accepted auditing
standards, consistently applied.  Landmark's obligations pursuant to this
Addendum are conditioned upon the results of that audit being in substantial
agreement with the unaudited revenues previously reported by MGA and its
subsidiaries to Landmark with respect to such period.

                                  ARTICLE 11
                                MISCELLANEOUS

       SECTION 2.1  ENTIRE AGREEMENT.  This Addendum and the agreements,
instruments and documents contemplated by this Addendum represent the parties'
entire agreement with respect to the subject matter of this Addendum and such
other agreements, instruments and documents and supersede and replace any prior
agreement or understanding with respect to that subject matter.  This Addendum
may not be amended or supplemented except pursuant to a written instrument
signed by the party against whom such amendment or supplement is to be
enforced.



       SECTION 2.2  COUNTERPARTS.  This Addendum may be executed in multiple
counterparts, each of which will be deemed to be an original and all of which
will be deemed to be a single agreement.  This Addendum will be considered
fully executed when all    parties have executed an identical counterpart,
notwithstanding that all signatures may not appear on the same counterpart.



        SECTION 2.3  SEVERABILITY.  If any of the provisions of this
Addendum are determined to be invalid or unenforceable, such invalidity or
unenforceability will not invalidate or render unenforceable the remainder of
this Addendum, but rather the entire Addendum will be construed as if not
containing the particular invalid or unenforceable provision or provisions, and
the rights and obligations of the parties will be construed and enforced
accordingly.  The parties acknowledge that if any provision of this Addendum is
determined to be invalid or unenforceable, it is their desire and intention
that such provision be reformed and construed in such manner that it will, to
the maximum extent practicable, be deemed to be valid and enforceable.



                                     -2-

<PAGE>   3

       IN WITNESS WHEREOF, the undersigned have executed this Addendum
effective as of the date first above written.



                                  LANDMARK GRAPHICS CORPORATION



                                  By: /s/ WILLIAM H. SEIPPEL
                                      ___________________________________
                                      Name:  William H. Seippel
                                      Title: VP, Finance & CFO




                                  /s/ FRANK D. MCMORDIE
                                  _______________________________________
                                  Frank D. McMordie



                                  Eppler, Guerin & Turner, Inc. as
                                  Custodian for the benefit of
                                  Frank D. McMordie



                                  By: /s/ MARK R. McCASKILL
                                     ____________________________________
                                     Name:  Mark R. McCaskill
                                     Title: Vice President



                                     -3-

<PAGE>   4

/s/ ELIZABETH McMORDIE-SCHMIDT         /s/ STEPHEN H. JAFFE
_______________________________        ____________________________________
Elizabeth McMordie-Schmidt             STEPHEN H. JAFFE
                                       Eppler, Guerin & Turner, Inc. as   
                                       Custodian for the Benefit of       
                                       Stephen H. Jaffe                   
/s/ JENNIFER L. McMORDIE 
_______________________________
Jennifer L. McMordie                   By: MAX R. McCASKILL
                                           ________________________________
                                           Name:  Max R. McCaskill
                                           Title: Vice President
/s/ CHRISTOPHER L. McMORDIE
_______________________________
Christopher L. McMordie
                                       /s/ THEODORE S. ROZSA
                                       _____________________________________
                                       Theodore S. Rozsa
/s/ HELEN LINDSLEY
_______________________________
Helen Lindsley
                                       Vendanges Investments, Inc.

/s/ GORDON R. BAYSTROM
_______________________________        By: RODERICK G. MUNRO
Gordon R. Baystrom                         _________________________________
                                           Name:  Roderick G. Munro
                                           Title: President
Eppler, Guerin & Turner, Inc.
As Custodian for the Benefit of
Gordon R. Baystrom                     Calyx Investments, Ltd.

By: MAX R. McCASKILL                       By: JAN MILLS
    ___________________________            _________________________________
    Name:  Max R. McCaskill                Name:  Jan Mills
    Title: Vice President                  Title: Vice President

/s/ GUY M. HUMPHRIES, JR.              /s/ JAMES R. LOVETT
_______________________________        _____________________________________
    Guy M. Humphries, Jr.                  James R. Lovett

Eppler, Guerin & Turner, Inc.                                                
As Custodian for the Benefit of        /s/ KIRK D. HANES
Guy M. Humphries, Jr.                  _____________________________________   
                                           Kirk D. Hanes         
By: MAX R. McCASKILL                                                            
    ___________________________                                              
    Name:  Max R. McCaskill            /s/ DEBORAH CLOSE
    Title: Vice President              _____________________________________ 
                                           Deborah Close                


                                     -4-

<PAGE>   5
/s/ GRANT GENEREUX                       /s/ IAN LEITCH
_________________________________        _________________________________ 
Grant Genereux                           Ian Leitch



/s/ DOUG SPACKMAN                        /s/ ROBERT PATTERSON
_________________________________        _________________________________ 
Doug Spackman                            Robert Patterson



/s/ DAVE SAVELLE                         /s/ KEVIN ROBINSON
_________________________________        _________________________________ 
Dave Savelle                             Kevin Robinson



/s/ GREG ROY                             /s/ BRIAN JANZEN
_________________________________        _________________________________ 
Greg Roy                                 Brian Janzen



/s/ JIM BURGLIN                          /s/ PRESTON NALDER
_________________________________        _________________________________ 
Jim Burglin                              Preston Nalder



/s/ DAN BEWS                             /s/ RICHARD WARD
_________________________________        _________________________________ 
Dan Bews                                 Richard Ward



                                     -5-

<PAGE>   6

                                 EXHIBIT 1.2

                                  ARTICLE 3

                             CERTAIN DEFINITIONS

       3.1   Certain Definitions.  For purposes of this Agreement, each
capitalized term listed below shall have the meaning set opposite such term
below:

       "Closing" shall mean the consummation of the purchase and sale of the
       MGA Stock and the ME1 Shares contemplated by this Agreement, the payment
       of the Class A Shares Cash Consideration, the Class B Shares Purchase
       Price and the MEI Shares Purchase Price, and the execution and delivery 
       of the other agreements and documents, and the completion, effectuation,
       and performance of the other acts and actions contemplated by Article 13
       hereof.

       "Closing Date" shall have the meaning specified in Section 13.1 hereof.

       "Earnout Payments" shall mean, with respect to each Earnout Period, the 
       amount set forth opposite such Earnout Period in the following table:

              Earnout Period ending June 30       Earnout Payment
              -----------------------------       ---------------

                        1995                        $ 1,808,095
                        1996                        $ 2,248,150
                        1997                        $ 2,843,755

       It is understood and agreed that notwithstanding the use of the term
       "Earnout Payments" the above-referenced payments are not in any manner
       dependent upon or to be determined with reference to the financial or
       other performance of MGA or any other entity.

       "Eamout Period" shall mean each applicable twelve-month period ending
       June 30, 1995, 1996 or 1997.

       "Eamout Recipient" shall mean each Person which was a holder of Class A
       Shares immediately prior to the Closing Date.

       "Person" shall mean any individual, or any corporation, partnership,
       trust, or other entity.



EXHIBIT 1.2 - ARTICLES 3 AND 4 - Page 1



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                                  ARTICLE 4

                               EARNOUT PAYMENTS

4.1   Payment of Earnout Payments.

      (a)    Payments.  The Earnout Payments for each Earnout Period shall be
paid by Landmark on the September 30 immediately following the end of such
Earnout Period (the "Earnout Payment Date") and shall be accompanied by a
Statement of Claim (as hereinafter defined).  Earnout Payments shall be made to
each of the Earnout Recipients in proportion to the number of Class A Shares
each Earnout Recipient owned immediately prior to the Closing Date.  Each
Earnout Payment shall be made by check or by wire transfer if requested.

      (b)    Adjustments.  If a claim is made with respect to the
representations and warranties set forth in Article 6 of this Agreement or the
covenants of MGA set forth in this Agreement, the remaining Earnout Payments
(net of all previous adjustments made pursuant to this Section 4.1 (b)) shall
be reduced by the amount of the claim (the "Claim Amount").  If the amount of
the remaining Earnout Payments (net of all previous adjustments made pursuant
to this Section 4.1 (b)) is greater than the Claim Amount, Landmark shall
(after such reduction) have no further rights against the Shareholders with
respect to such claim.  If the Claim Amount exceeds the amount of the
remaining Earnout Payments (net of all previous adjustments made pursuant to
this Section 4.1(b)), the Warranting Shareholders shall pay such excess to
Landmark pursuant to their respective Indemnity Agreements (as hereinafter
defined).

      4.2   No Assignment of Earnout Payments.  The right to receive
Earnout Payments shall not be assignable except by will and the laws of
intestate succession, or otherwise by operation of law.

      4.3    Statement of Claim.  In the event Landmark asserts a claim
pursuant to Section 4.1, Landmark shall promptly provide each Earnout
Recipient and the Sellers' Representative a statement (a "Statement of Claim")
describing its claim and the method of its calculation.  Any such Claim
Statement shall also set forth the effect of such claim upon the remaining
Earnout Payments.  If within thirty (30) days after delivery of the Statement
of Claim to the Earnout Recipients (including Sellers' Representative),
Sellers' Representative or one or more of the Warranting Shareholders has not
given written notice to Landmark disputing such Statement of Claim and
indicating the basis of such dispute, such Statement of Claim will be deemed
to be conclusive with respect to such claim and its effects upon the remaining
Earnout Payments.  In the event Landmark receives such notice of dispute
within such 30-day period, Sellers' Representative and Landmark will use their
best efforts to settle the dispute within thirty (30) days after the giving of
such notice.


EXHIBIT 1.2 - ARTICLES 3 AND 4 - Page 2