This is how I started myself, 2 computers, 2 accounts, mainly to assist my first character.

What I did in the beginning, was to make a macro for follow (/follow party1) when I grouped them up, and then used /assist macros to allow for attacks etc. I would then have the two keyboards above eachother, and simply hit my hotkeys on the 2nd keyboard to do stuff on the 2nd account.
Thanks to this website and the forums here, I have now "advanced", by making better macros and using a program called AutoHotKey to send the same keypresses to all my wows on the same computer.
There is also a program called synergy, which allows you to distribute keypresses to other computers on a network, but I was never successful in setting this up. With a reasonably decent computer though, you should be able to run 2 wow's off the same machine.

Myself I am currently in the process of escalating to 5 boxes, and have registered 3 trials to see if my computer can handle it, before I go out and buy the actual boxes.