Ok, I'm not at my REAPER right now, and it's not something I ever do, but from memory....
Let's say you want to bounce Tracks 1 & 2 to>>>>Track 3.
1) Solo tracks 1 & 2
B) Record Arm Track 3
3) Right click on the record button on Track 3 and select "Record Output".
D) I think you can also choose if you want to record to a stereo track or a mono track. If you want a stereo track, make sure to pan the source tracks (1 & 2) where you want them to end up in the stereo field.
Man, I hope I'm not steering you wrong. If I am, I'm sure someone will correct me. But, for now, it won't hurt anything if you follow those steps and see what happens.