Paul, Silvanus, and Timothy, to the assembly of the Thessalonians in God our Father, and the Lord Jesus Christ:
Paul is the author of this epistle, not Silvanus and Timothy. These two men of God are called simply co-laborers. It is an act of courtesy and display of friendship for Paul to include them. They are both well known, loved, and appreciated by the Thessalonians; and Paul respectfully includes them in the salutation of this letter.