Hi there! I've been looking into PhysX lately, while trying to decide between an Ati or a Nvidia card for my new system and I found that it is indeed possible to use an Ati card as your primary GPU and use a Nvidia card a PPU at the same time, what you need to do is to install some modified drivers. Check out this page:http://www.ngohq.com/graphic-cards/17706-hybrid-physx-mod-v1-03-v1-04ff.html
It looks like a pretty solid mod but you'll have to try it out yourself to decide if it works correctly.
(This drivers are just the ones I found, there may be some other different solutions out there or a different page with more current versions, it's up to you to do some more research).