One of the problems with that is the lack of common ground for DC. That can cause some rather serious damage, over time, to the electrical components in your computer.
Everything in your computer is connected to the motherboard. The motherboard ground plane carries the electrical energy to the various power supply ground lines. You would not be able to incorporate a second power supply due to the lack of common ground.
Even if you managed to get common ground, you would have to make sure the ground was carrying the correct power levels or you could burn out the smaller supply.
In a nutshell. It's more trouble that it is worth.
