If you just want the "playback of both" C+Bb at the same time...
I think the easiest way would be to make arrangement in C and copy these arranged RT's to Utility tracks, if you have 2021 BIAB, if not there is a thing that is called Performance track, it just dumps mixed arrangement to a separate track... Utility tracks (and performance track) will not follow what is on the chord sheet if you make changes later, but will stay at your "C", but they will play in sync (start/stop) if you rewrite arrangement in Bb and press Play, both original C (that you copied to Utility tracks or Performance track) and newly made Bb should play at the same time. If I understood you correctly and this is the goal.