When I am running 4 graphic cards in quad SLi I have to run three PSU's to feed the hungry beast that is the 580GTX.
So if you are asking can you provide power to other graphic cards via a second or third PSU yes.
The PSU is only providing current when switched on. I know it is obvious just wanted to be clear on that one.
So if you are running multi cards and feeding power to two of them via a separate PSU both PSU's need to be powered up for the PCIe connectors to work.
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