12 that is, that you and I may be mutually encouraged by each other’s faith.

Read Romans 1:12 Using Other Translations

That is, that I may be comforted together with you by the mutual faith both of you and me.
that is, that we may be mutually encouraged by each other's faith, both yours and mine.
When we get together, I want to encourage you in your faith, but I also want to be encouraged by yours.