Besides Timothy, who is named with Paul in the opening greeting of 1 Thessalonians?

Answer: Silvanus

The letter is from "Paul, Silvanus, and Timothy, to the church of the Thessalonians."

“Paul, Silvanus, and Timothy, To the church of the Thessalonians in God the Father and the Lord Jesus Christ: Grace and peace to you.”— 1 Thessalonians 1:1 (BSB)