x2vnc will let you use two screens on two different computers as if
they were connected to the same computer, even if one of the computers
runs Windows and the other one runs X on *nix.  

This requires a vnc client such as TightVNC (available at SlackBuilds.org)
in order to be useful, but there's no compile-time requirement for one.

See the x2vnc homepage and x2vnc(1) for usage information.