x2vnc 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.

Homepage:
https://fredrik.hubbe.net/x2vnc.html
