When I am connected to a Debian server through x2go, my X applications work.  When I su - user2 then try any X application they don't work.  For example emacs will start in terminal mode and java applications give errors such as
java.lang.InternalError: Can't connect to X11 window server using ':52' as the value of the DISPLAY variable

When I echo $DISPLAY before the su -, :52 is the correct display.

I have several other machines that do not have this problem.  How do I fix it so that X works through x2go after su -.