I dumped as close to your system as I could approximate (guess) here: http://extreme.outervision.com/PSUEngine
With OC'd CPU at 4.7, 2 6870s, 4 HDDs, 1SSD, 4 modules RAM, an optical drive, a few PCI-E cards, 4 case fans, 100% load on all components, it estimates ~730w power draw from your PSU. A single 850w PSU would suffice, but if you're fully loading everything at the same time (unlikely since you're not crossfiring--assuming only one card load at a time it would be ~625w full load).
You can probably better-approximate your system than I could, since I was just guessing.
Personally, I wouldn't chance bridging two 500w PSUs when a single slightly higher PSU (than 500w) would suffice.