Dual boot drive with only one user acccount?

After much wrangling with terminal and the diskutil command, I have finally got my 3 TB fusion SSHD formatted for APFS, installed Mojave and successfully installed Windows 10 on Boot Camp. Huzzah!

Flushed with victory, I followed online directions and added a volume to the HD container for Catalina, and installed that, too. I now have a triple-boot 2017 iMac! Couple of questions though....

The users of Mojave and Catalina are the same (I mostly use Mojave, my son mostly Catalina); is there any way to have shared user profiles across the two installs? Would save a lot of space and be much more convenient.

The Catalina volume has split itself into the original, and a Data portion. I ran into this during my disk wrangling, and want to make sure I can re-fuse the two Catalina volumes without disturbing the original Mojave?

Mojave 10.14.6 (Retina 5K, 27-inch, 2017) 4.2 GHz Intel Core i7, 32 GB 2400 MHz DDR4, Macintosh SSHD Radeon Pro 580 8 GB