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.
For a while there, companies were making power-supplies that did just this. They would fit in your computers 5.25" bay and would provide an extra 200W+ for PCIe-graphics cards. Something like this (note that they no longer sell it) http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Like reccy said, if you did add another PSU, you have to synchronize both PSUs so that they power-on at the same time, which could be difficult.
However, if you are going to buy an entire second PSU to power only the GPUs, why not just buy 1 PSU that's powerful enough for the whole system?