This is a mere optimization of seperately signing t1 and t2.
Dual signatures save time in that only one signature operation is required instead of two.
The dual signature also links the two messages together. The signature itself links the two messages together since there is a hash of each component in the signature. With the prior definition of SET, cross-substitution was possible without reversing SHA because the sender of the message supplied the dual signature to each recipient independently, that is, the cardholder sent one dual signature as part of the OI to the merchant and another copy as part of the PI to the payment gateway. With the May 31st version of SET, the dual signature was moved outside of the encrypted portion of the PI and now one copy is used by both the merchant and the payment gateway. Therefore, the problem of cross-substitution has been eliminated by a minor movement of data in the SET message.