The verb which Paul uses in this verse (joins himself … to declare) occurs also in 2.15 and appears again in 9.1. In the present context the verb may mean either “to bear witness with” or else “to confirm another’s witness.” Most translators take it in the former sense. In some languages this meaning of “bearing witness with” may be translated as “God’s Spirit says at the same time with our spirits” or “God’s Spirit and our spirits both say.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on Paul’s Letter to the Romans. (UBS Handbook Series). New York: UBS, 1973. For this and other handbooks for translators see here .