HI，
short answer is yes，you can have both emacs displayed in GUI
i think the problem is when you switch to goo，the auth to DISPLAY is lost
i guess you login to foo with display-manager such as gdm or something alike，and switch to goo with su
serveral ways
a little tricky but simple to work
1. switch to goo via ssh -X goo@localhost
2. try to start emacs in this session
the 2nd one is to use xauth to deal with X permission
1. as foo
echo $DISPLAY （typically you will get :0）
xauth list （remember the hexcode ）let's say $hex
2. as goo
export DISPLAY=:0 （or what you actually get in 1）
xauth add :0 . $hex （easiest way is just do a copy-and-paste）
3. try to start emas in this session
you can also use “xauth merge” or “xhost”，but the principle is the same
hi. in debian 9 i start emacs as user foo from the terminal and the
gui start up. i start emacs from the terminal as another user goo and
the text version comes.
Can both users have their own gui emacs sessions ?
Alexandros