Hi guys, you may use any number of psu but you have to connect all the ground together (i mean the black wires).
I use an atx psu for motherboard, hard disk and risers. Then i have a second psu taken from a server (750watt - 70A on 12V) just for the vga.
Then connect 1 black wire from psu 1 to psu 2 and the voltages will be good.

If you don't short them you may test the voltage between the grounds (-) ... it could be 0.2-0.3 volts and that's not good.