A spur of the moment, not well thought through thought:
Perhaps using two simultaneous instances of (portable) MusicBee would do the trick?
(having 'continuously monitoring' activated for both)
I would describe this as A Bad Idea.
MusicBee isn't designed for concurrent use, so you could frequently lose updates to settings, tracks, tags, etc. if one instance overwrites the changes made by the other while both are running.
Google "Database Lost Update" for a more detailed explanation.
I'm sure a lot of people would find it useful, so I suggest posting a formal request on the Wishlist forum.
Like many similar requests though, I see this belonging in the realm of 'easy to picture, hard to implement', so it probably wouldn't be done soon.
FWIW, I'm always moving files from working folders into my main music Library folder too, so I do all my file management using Directory Opus which has dual file views (and a lot more besides).
That is a paid program, though there are lots of free File Explorer alternatives with dual folder view support.
A quick search shows OneCommander, FileVoyager and FreeCommander.
(Not an endorsement - I've never used them)
Acknowledging too, that a lot of users would rather pluck out their eyeballs than do file management outside of MusicBee, but it's fine if you know the implications. I've done it for years without any problems.
Bee excellent to each other...