Isn't the default installation of Ubuntu to BTRFS? In which case, you should have an @ subvolume with Ubuntu that's mounted to /, and an @home subvolume that's mounted to /home.
Make a new subvolume, install a new operating system into it, and choose that subvolume in the bootloader, should be able to have Ubuntu and 'your favourite OS' (I use Arch btw) living side-by-side with the same home directory.
More "the Human Centipede".