If the two computers are within about 10-15 feet of each other, you could pick up a 2 port KVM switch (Keyboard, Video, Mouse)for about $20-$40 bucks. With a 2 port KVM switch you only need one keyboard, one mouse, and one monitor to run two computers. With most KVMs, all you have to do is just tap the "Scroll Lock" key twice to change from one computer to the other.
NewEgg.com
CompUSA.com