X crashes when closing one of two running X sessions

[Robert Arkiletian <robark@gmail.com>]

Using Debian Stretch x86_64. I don't use a greeter like lightdm. I
just log in at the virtual terminal, then startx (xfce desktop). I
like to use multiple accounts (running X) at the same time on
different virtual terminals (eg. ctrl-alt-f1  ctrl-alt-f2)

All is fine until I exit (shutdown) one X session, then the other
running X session freezes and then after a little while crashes. Any
ideas on how to find or diagnose what's causing this issue are much
appreciated.

Thanks