If you have two computers near each other, you may want to invest in a KVM switch. A KVM switch allows you to let two or more computers use one keyboard, one monitor, and one mouse (hence KVM). This will save you money and space. Most provide both keystroke and hardware (pushbutton) operation. Having four computers without a KVM results in keyboards and mice that can get quite messy.
This is an IOGear four-port KVM switch, just one example.