Its dangerous to do that. I'd suggest getting the correct power supply that can handle all. If you were really good with electronics and able to get both power supplies perfectly balanced ( same exact voltage so they are equally balanced ) it could be done, but if not balanced it wouldnt be good... potential smoke show!
Most PC power supplies dont have a POT in them to adjust their voltage as well and so you could have one power supply at say 12.2VDC and another at 11.5VDC for the 12Volts you would need for the video card and this would be dangerous with a .7 volt offset wherever part of the computer is assumed that the 12VDC comes from the same source and now its not from the same source. A tiny trace could then try to carry the current to bring another part of the system up to 12.2VDC where the far end of the circuit is only getting 11.5VDC and you then would burn up a trace possibly.
I suggest doing it right and just getting a heavier power supply that can handle all, as for its dangerous to team them.
