There's two ways which will keep/set the DISPLAY variable:
First, if the DISPLAY variable is set in user1 and you do "su user2",
then the environment of user1 will carry into user2. IOW, you're good.
Second, if you, instead, do "su - user2", then you will have a fresh
environment, that of user2. If user2's environment doesn't have the
DISPLAY variable set properly, you can do this by setting this in
user2's ~/.bashrc file... if user2's default shell is bash-- if you're
using some other shell, consult the manpage for the proper file and
command syntax for setting the environment.