You can accomplish your goal in 3 ways: hardware, software, or just use one computer.

Using one computer would be the easiest. Have you tried running 2 copies of wow on one? Chances are if you have even a half-decent computer it will run smoothly. Then all you need is keyclone or autohotkey. Keyclone is much better (easier) than AHK, but it costs $20.

To do it with hardware is next easiest. Buy 2 wireless keyboards. Connect a receiver to each computer, and then use one keyboard to connect to both of them.

The software way is most complex, and since I have never gone this route, I can't give you detailed advice. Buy 2 copies of keyclone and set it up to relay your keystrokes to the other computer.