permissions - When I create two users with the same UID and GUID but different home directories, my 17.10 Ubuntu fails to boot, why?

I want to be able to log in with different settings but still be the same "user". So, I added a 2nd user with the same UID/GUID but different home directory and now when booting Ubuntu the system just hangs. If I remove the 2nd user from the passwd and shadow files, the system boots just fine.