Currently I have a 600w PSU and a 560 ti graphics card. I'm considering getting another 560 ti in the next few months and going SLI, but the 600w isn't going to be enough. I have another 600w power supply laying around, different manufacturer, and I've seen that it is possible to run two power supplies to one computer here.
I'm wondering if it is ok to have one PSU run one of the graphics cards, and the other PSU run the other graphics cards. So it would be a different power supply to the each graphics card. I don't see how this could cause a problem unless one PSU didn't turn on, using the push on push off method explained in the link I would be able to turn the extra power supply before booting up my computer. Would this be able to work or is there a problem I'm not thinking of?
All you need to do to power on a power supply is jumper the green wire to any black wire with any device connected that can put a load on the power supply without much sensitivity to power on/off cycles, a CD ROM drive is good. So by attaching a CD drive and one video card to the secondary power supply you can effectively reduce the load on your primary power supply.
Also, instead of making a switch for your second power supply, strip back the green wire and one black wire on both power supplies and jumper the corresponding wires together and connect one of the 20/24 pin to the motherboard, this way the two power supplies will power on and off together.