In APFS two boot containers (10.15.x on one and 10.14.x on the other) that share a common data container?
I would say you need 2 systems and one third volume to do this the right way. It’s long been possible and sometimes preferred to have your user home folder apart from the boot system.


10.14 has a combined default user data and system - add your main user with a home folder outside that, but shared with APFS so that all the free space combines.
10.15 has a read only system and a paired user data volume with firm links. Add the same main user outside these volumes.


You will certainly have some breakage and possibly much breakage when you use things like Photo Library, podcasts, music and more when the 10.15 system reads the data structure and modifies / migrates then in the user folder on disk to a newer structure. When the OS boots back to 10.14 this breakage will show and depending on your needs, you might need to keep two parallel home systems, so I would make a good backup of the 10.14 side before you boot to 10.15 and open anything more complicated than isolated documents.