I once put a pair of 550W PSU's in an old server case and ran the second just for my SLI ... as I recall it was a pain to get the second PSU to fire up and run the cards on it's own.

jsc can you remember how to jumper an old ATX PSU so it powers up without attaching the 20 / 24 pin plugs to the mobo?

I can't remember what I did.

Alternatively get a huge single PSU or one of these (which I think is what the OP is asking for):

http://compreviews.about.com/od/cases/a/SupplementalPSU.htm
 

shovenose

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you put a paper clip between the green pin thats sort of in the middle of the 20/24pin power connector and using a piece of wire or paper clip and connect it to the a black one..or solder it...also, theres a little cable sold especially for that purpose, but i dont remember what its called and what the webaite for it is.
 

shovenose

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thanx for adding the picture, but i use a paperclip...anyway, pity i cant remember the link for the little cable that you put between the main psu and the mobo, and then it has a little thingy that connects to the green and black pins on the secondary psu so u dont have to remove the little wire everytiome u want to turn off the pc!