You probably need to give a fair bit more information on what exactly you want to do, what parts you have ect ect. Most of the members here cant read minds.
Well if you are just sharing a monitor, they aren't acting as one. They are just sharing peripherals such as the monitor, keyboard, and mouse. It's called a KVM switch.
I think I know what he wants to do. He's basically looking to double his computing power by running the two computers together. I think he'd need something like a remote desktop setup.