Re-trying to use APFS volume groups for dual boot reminded me why I reverted to the old partition scheme. Long story short:
"It is not possible to bless another system on the same APFS volume group".
My setup for this is somewhat special so here are the details for those who are interested:
I use an external disk as a test bed for Catalina and occasionally also Mojave. It is partitioned as 1. Catalina-clean (install), 2. Catalina-test, 3. Mojave-clean and 4. Mojave test. I can quickly reset the test partitions to a clean install state by cloning the clean partition to it with Carbon Copy Cloner (actually I usually clone it from a locked CCC disk image).
OK. Making APFS volume groups was very handy and unlike old partitions, as a great bonus, it left unused space on the small 320GB HD shared between all those volumes (it is an old slow spinning 5400 SATA HDD kannibalized from an old Asus laptop. I am cheap).
But where this seems to fall apart is that when I am booted from the Catalina-clean, CCC refuses to completely clone it (or from a disk image saved on another external disk) to the the other APFS volume (Catalina-test) because:
"It is not possible to bless another system on the same APFS volume group". In Mojave CCC seems to successfully clone all files but the blessing must be done while booted from elsewhere. But CCC seems to alert that much of Catalina content can't be cloned then.
Anyway, maybe I continue with APFS volume groups because it seems to handily share unused space for all volumes. Maybe I could tweak the APFS volume groups on that same disk so I can boot from another group to do the blessing? Or add an old-style extra partition for Catalina so I can bless the APFS Catalina volumes (or if that fails, use some other spare disk or flash drive to bless the Catalina volumes (this is my one and only Catalina install, Mojave being my main system).