Is one of the systems a Mac? Then look at air display.
Ever wish you had an extra display for your computer? There’s an app
for that! With Air Display, you can extend your desktop wirelessly to
your iPad, iPhone, iPod touch or Mac as a wireless display.
If you are using Linux then DMX may be what you want.
Xdmx is proxy X server that provides multi-head support for multiple
displays attached to different machines (each of which is running a
typical X server). When Xinerama is used with Xdmx, the multiple
displays on multiple machines are presented to the user as a single
For Windows look at Maxivista.
MaxiVista allows you to use a Laptop as a second monitor for your
primary computer. No extra multi monitor hardware is required. Simply
extend program windows across multiple screens as if it were one big