Re: How does one add a second Ubuntu installation to anexisting one?

[John Little]

Rather than juggling partitions, consider using a btrfs. I typically have four 
or five different *buntus in separate subvolumes.

Also, I suggest also installing grub to its own subvolume (or partition), and 
manually editing grub.cfg there, using the vmlinuz and initrd.img symbolic 
links so that the entries don't have to be updated frequently. I suggest using 
partition labels in /etc/fstab and grub.cfg rather than UUIDs; they're much 
more readable. 

If multiple distros have grub installed to update the boot, each tries to 
control the boot and confusion is the result. Consider switching to the grub 
package that does not automatically update the boot, at least on all but one 
install, or none as in my suggestion above. 
--
Regards, John Little