Clean up snapshots with unplugged target.

Hi!

I have a setup where I make a backup with similar schedules on two USB disks; one at home, one at work.

I am never making a backup with both disks plugged (unless I decide to do so). It seems the snapshots are always increasing. I understand this has to do with btrbk needing to know what are the latest snapshot/backup pairs?

I wonder if there is a setup that would be better for me then? Or if it would be possible to mark the snapshots to belong to a disk somehow so that it is still possible to prune enven with only one of the target disk present?