Once Extramon is installed and running, you can operate both computers from a single keyboard and mouse. Either one will work. You'll probably want to move the extra hardware out of the way to free up some valuable desk space.

When you press the Switch window to switch from one computer to another, your keyboard and mouse automatically transfer over to that computer.

You can also switch from one computer to the other just by moving the mouse off the edge of the screen towards the other computer. This is useful if, say, the left monitor is showing computer A and the right monitor is showing computer B.