Nvidia's Physix won't work in a mixed video card environment.
This what I heard as well, which is really lame IMO. It's not the exclusivity that bothers me but the fact that you can't use it even if you paid for it. I would love to run a beefy 7870 alongside my 560 Ti for physx, cuda, or even switching for a game in case of faulty driver support.
Intel Core i7-5775c, Asus Maximus VII Formula, Win 10 Pro
EVGA GTX 1080, Corsair 2x8GB DDR3-1866, Corsair AX860
Corsair H105, WD Red 4TB x2, Creative X-Fi Titanium HD
Samsung 850 EVO 1TB, ASUS ROG PG348Q, Corsair 450D