I think they mean, as like a 2nd PSU powering the PCI slot from an external source. As in 1 PSU powering the system and a 2nd PSU/Power Source powering the GPU/PCI-E Slot.
But to the above from my side of things, i think you can but ALOT of re-wiring from one PSU to the other to get the power to surge on at the same time would be necassary. I myself am not intelligent enough to figure this out or have the mind to actually do it.