I'm currently on using 16.04 on my daily-driver host. I am contemplating an upgrade to 18.04, but don't want to take the leap without test-driving 18.04 for a bit. So my question is: Can I run 16.04 and 18.04 side-by-side, choosing at boot which to use? I'm using EFI boot and Grub 2 at the moment (the default grub for 16.04).
Yes, on different disk partition.
Technically yes, but it can be dangerous, because it contains also settings for software. It can cause problems if different versions of the same program are on different OSs. There is workaround explained here in the first post where is created folder with home folder content and then this folder is accessed from second OS.