Personally I've never even considered trying this, but apparently it works out ok. The folks over at Hardware Zone even fashioned a clever Y-shaped ATX power cable for the project. The two system's aren't any kind of powerhouse(no SLI), but you'd be surprised just how little power is being consumed.
"If you've wondered if it's possible to run two systems with a single power supply unit, then the answer would definitely be a YES. We took it further with Intel's Core 2 Duo processors and running them silent with passive heatsinks. Find out more about this Computex Taipei 2006 technology demo at MSI's booth right here."