You need to enable X11Forwarding on the server box
Just setting the DISPLAY will not work if X11 is not forwared.
You also need to tell it you want X during the connection by specifying the "-X" flag
ssh -X firstname.lastname@example.org
But if you really think about this, it is the reverse of what you need
If what you really need is to control a remote display and display it on both machines at the same time, then x11vnc is what you need
we have packages for that in the repos.
It even comes with a nice gui to setup the server.
X11vnc allows you to display the currently running display on a remote box, and control it from there too.
That is if you're not concerned about security