@vanillaos, It seems like you are trying to reduce the partition size from 20GB to 10GB for each of the A and B root partitions.
Why not check out BTRFS snapshots, though it might be similar to the implementation of openSUSE's microOS?
But I think BTRFS snapshots are much more efficient.
You can also make it read-only in your case.
My default OS is Garuda Linux, which implements BTRFS' snapshot capabilities and it makes the arch experience rock solid stable!
And this distro was literally what stopped my distro hopping (its selling point- BTRFS snapshots).
I think you should check it out.