This program merges the capabilities of x2x and vncviewer.
It will allow a machine with an X display and a machine with
a VNC server running on its main screen to act as if they
were two displays connected to one machine. When you move
your mouse pointer off the screen in a direction of your
choosing, the pointer will appear on the other screen instead.

WWW: http://www.hubbe.net/~hubbe/x2vnc.html