Two Systems with One PSU?
A lot of users often asked if it was ever possible to run two systems with a single power supply unit. While some users reported success, it wasn't something that we would encourage since most of today's processors would be too power hungry. It may still be possible to power up two of the latest Intel Pentium D or Athlon 64 X2 systems but you may need to get a pretty powerful power supply to get it working.
Fortunately for today, both AMD and Intel have introduced new processors that are less power hungry. Intel's upcoming Conroe processors (known as Intel Core 2 Duo) would draw only about 65W while AMD's low power Athlon X2 processors that were recently announced would draw 65W for the Energy Efficient model and 35W for the Energy Efficient Small Form Factor model. These processors are powerful dual core CPUs which gives you a lot of mileage in terms of compute performance and would make great machines for all sorts of purpose, be it gaming, video rendering or even a workstation.
In order to get two systems running from a single power supply unit, one would need to construct a customized 20-pin ATX power cable with a single female connector on one end and two male connectors on the other. This would multiply the regular 20-pin power cable in your power supply unit to support two motherboards. We went to a regular electronic hardware store and purchased a couple of male and female connectors along with the pins that go along with these connectors. Constructing the cable is just a simple and long-winded process of crimping the cables together. 
To really quicken the process, we took two old 'about-to-discard' power supplies and cut out the ATX power cable. This will give us the ready made end of the two male connectors along with corresponding color coded wires. The next step would be to match the cables pin for pin and have them crimped with the pins. You just have to make sure that all the wires are crimped correctly (e.g. pin 1 should go to pin 1) and you'll have a Y-shaped ATX power cable as shown in the picture below.
Stacking Up the Two Systems
We then took two of MSI's latest Intel P965 full-ATX motherboard and have them installed with Intel's latest Core 2 Duo processors running at 2.66GHz. In order to make the stacked systems in a smaller profile, we removed the fans from the coolers. We also used smaller half height graphics cards so that the systems would require less space. Hence, we picked MSI's GeForce 7300 GS graphics cards and removed the I/O bracket. The following series of pictures would give you a clearer idea of how the two systems are stacked. 
Here's a couple of things that one should note :-
- Since we did not use fans on the processor coolers, for reasons of building a really silent PC, we recommend that you install a 12cm chassis fan right beside the two processors. The processor may feel hot at full load, but it should still be within operating limits.
- We would recommend using a heatsink with straight fins aligned along the direction of the chassis fan's air flow direction. This would be a more ideal cooling strategy for such a setup. Of course, radial heatsinks are way much nicer in terms of appearance.
- We picked 2.5-inch SATA notebook drives because we think they are neater. Of course, if you want larger storage capacities, a full size 3.5-inch hard disk drive would be suitable too.
- If you want better airflow across the system, we recommend that you use a board with integrated graphics.
- This setup could also be achieved with AMD Athlon X2 low power processors too, not necessarily one that's based on Intel's latest Conroe processors. It's just that Intel's Conroe processors were more readily available than AMD's Energy Efficient processors right now.
- You can connect the Power On headers of both the boards to a single switch, so that they are both turned on at the same time.
So, how much power does the entire system draw? We measured it using Cooler Master's built-in power meter and it came out to be a little over 200W. Pretty amazing, isn't it? This gives users a lot of headroom for other more powerful devices.
Now, for those wanting to explore clustering (using OpenMosix on Linux), this would be an ideal setup. Of course, users can do anything they want with each of the two systems as the types of usage would only be limited to one's imagination. Now, we just need a funky casing to go with it....