APFS, two data volumes on a single OS partition for dual booting
No, the firmlinks are bidirectional so you should have two pairs of filesystem if you need separate systems. The good news is that with space sharing and the smaller size of the system, having two Catalina around is ~22 GB for my Macs. 


https://eclecticlight.co/2019/10/11/macos-catalina-boot-volume-layout-revised-for-10-15-release/


See the red boxes in the PDF for all the firm links you would have to remap and contend with...