Bedrock Linux does indeed support workflows with multiple copies of Debian, and this does in practice often resolve the quoted concern of mixing parts from different Debian releases without having them step on each other problematically. Another use for having multiple Debian subsystems (what Bedrock calls "strata") is to serve as a low risk alternative to an in-place dist-upgrade: If anything doesn't work as desired in the new Debian release, just keep using the previous one until you get it working; once everything's migrated over, you've effectively dist-upgraded and can (if you wish) remove the old stratum.
I'm Bedrock's founder and lead developer. If you have any questions about it I'm happy to answer them.
I'm Bedrock's founder and lead developer. If you have any questions about it I'm happy to answer them.