Re: Dual Root.
However, if that is so obvious, why nobody thought of that before?
This was how you upgraded the OS of a mainframe I worked on in the 1980s-90s. Upgrade a partition, boot from it. If all is OK continue, else reboot from old (good) partition.
I have not so fond memories of an upgrade that dumped half the new files on the 'new' partition and the rest on the 'old' partition. Followed by boot from 8" floppies, in order to be able to restore a HD partition from backup....