The "minimum requirements" for a 6950 is a 500Watt power supply... they pull about 169 Watts @ load with stock settings.  I'm running an XFX 6950 @ stock and netting 318Mh/s with a Corsair 600 Watt PS with zero problems.  I wouldn't be scared to add a second card if it had the extra PCI-E connectors.

I question that 8% variance though... the 6970 has more shaders and a higher clock speed... you might be comparing the 6950's with unlocked shaders.  Those cards are getting a little harder to find.... my XFX has a locked bios and I may not be able to unlock those extra shaders (people have been having a 50/50 chance on this particular board revision).

If you do have unlockable 6950's though, you basically already have 6970's.