You can install your 6950 in the same system with your dual 670 an still use PhysX. That's because the 670's support PhysX. As renz496 said, PhysX is an Nvidia technology, period! Nvidia thought it up and they implemented it on their GPU's. AMD has there own technologies and since AMD graphics cards compete directly with Nvidia graphics cards, they DON'T share!
I am trolling ofc, thanks for paragraph detailing what the guy already said lol.
Was bored made post
Perhaps it detected the AMD card in the nvidia control panel.
But as of yet, there is no decent hack or mod that allows PhysX to work on AMD graphics chips. Or Intel for that matter. (I would LOVE to use my HD4000 for PhysX)
Trust me, I've looked. There just really isn't a driver-hacking community like there used to be back in the Geforce FX days.
Gone are the days of daisy chaining voodoo cards for miles ;3