Oh wait...I think I know what you are saying.
If you are using audio bus or IAA and midi or like a plugin, yes only one at a time can use it. You have to bounce the track to audio (solo, mixdown, import back into the project). Kind of like what freeze does but freeze doesn't release the IAA or audio bus insert.
IAA to me was harder to work with for that reason and AudioBus at least you could have the recorded wav to work with. But you will have to bounce to wavs to reuse the bus.
It isn't perfect...kind if reminds me of the bouncing in the 4 or 8 track tape days to get more tracks. Just have to commit!
Well and not really fully commit. You can always reroute, make the changes and bounce again.