I think it will work if you install in two different folders, e.g. install 2020 in a folder named: '\bb_2020\' and install 2021 in the normal '\bb\' folder.

Then in the \bb_2020\ version set the RealTracks and RealDrums to share the folders from the 2021 version. This is done from Options > Preferences > RealDrums > Custom Drums Folder.
Do the same for RealTracks.

I believe that you are correct in your assumption that the 2021 styles and RTs will not work with the 2020.


BIAB & RB2026 Win.(Audiophile), Windows 10 Pro & Windows 11, Cakewalk Bandlab, Izotope Prod.Bundle, Roland RD-1000, Synthogy Ivory, Session Keys Grand S & Electric R, Kontakt, Focusrite 18i20, KetronSD2, NS40M, Pioneer Active Monitors.