Thank you VideoTrack,
It's a solution but, the workflow with going to Real Band feels slow to me. Here's my workflow:
1: I like to get the bones of a song (bass, drums, guitar/piano etc) to work out the structure in BIAB.
2: I then add another instrument (as a fills/complimentary sound). There's all my BIAB tracks gone

3: I then port all tracks to Reaper
4: I then return to BIAB, remove one one of the tracks, add another to see what fits, if good, export back to Reaper
5: Rinse, lather, repeat...
Going back and forth like this is very tedious. More tracks in the BIAB window would be great.
I thought the originator of this thread was proposing this could be done. Am I wrong?
Thanks again..