Install this on the one that needs to be controlled:
http://www.redstonesoftware.com/vnc.html
Install this on the one that needs to control it:
http://sourceforge.net/projects/cotvnc/

And then connect the two with either a crossover ethernet cable, or a firewire cable.
Either one can be setup for networking from the network configuration pane.

Whichever one you use, setup a network between them using private ip addresses...
ie. machine 1 = 10.0.0.1 , machine 2 = 10.0.0.2 , netmask = 255.255.255.0