There are a couple of methods.
1. Have to computers side by side. And you can new work them too. I use this method as I have my Screw Around CPU acting as a DVD/DivX player from time to time.
2. Dual Monitor Video Card and 2 monitors. Really good for keeping an eye on things or working with documents at the same time. Eg. Me at work where i need one monitor to show price list, the other to show the company website. But don't try playing a game on one and expecting the other monitor to do some one thing CPU intensive. You are asking for trouble there.
3. KVM Switch. Cheapest and most space saving method. How ever you can only view 1 monitor at a time.
Sorry, I am recovering from a hang over right now, and the lateral thinking part of me as just taken over
what i meant by having 2 computers work at once was having them side by side because i read somewhere in here that it was better to have them side by side than to have 2 processors.