Subprojects can be used for this, so this is already quite possible.
While it's admirable you're checking out the manual, there's tons of info manuals don't cover, you should also probably go through Kenny's video tutorials here:
https://www.reaper.fm/videos.php
There's a lot of them. There's one for subprojects, too.
Also, perhaps it would be worth it for you to post in
this subforum first, asking if something might be already possible in a way that manual doesn't cover, before you start rapid firing new threads in feature requests forum?