Supplemental power supplies, for video card.

I just bought a 850W Zalman power supply for Tri- Fire 4870x2 and 4870. But I have seen incredible benchmarks of Quad-fire 4870x2 and now I might go that route. Obviously my 850W can't handle 2 of those beastly cards, but instead of upgrading once again, is it safe or practical to use a supplemental power supply ?
http://www.newegg.com/Product/Prod [...] _-17104019
with this supplemental power supply, I could power both of the cards 6 pins, and power their 8 pins with my Zalman. Is this a good idea? Does anybody use these things? I have heard little about them. Thanks

I have heard that 4870x2 quadfire will require at least 1000W.

Here is another supplimental power supply. http://www.newegg.com/Product/Prod [...] 6817104054

If you're going to use an extra power supply, why not power one card with the extra psu, and the other with your main psu? It accomplishes the same thing but something about supplying each card with 2 different power supplies gives me an un-easy feeling.