1. Run ScreenRecycler
on the computer you want to to act as an additional display
2. On your other Mac, run the JollyFastVNC (or some other VNC client like Chicken of the VNC) and select the Mac running ScreenRecycler.
That should be all there is to it. ScreenRecycler supports Bonjour, so it will detect your Macs on the network easily.
Not sure what you meant by "cable," though. But because ScreenRecycler is a network-based application, you'll have to rely on getting your Macs on the same network.