You don't need a PhysX card to supplement a 680 even if it did work.
I don't see any reason it won't theoretically work, although there may be a practical reason that I don't know since I am not a huge expert on this.
In any event, PhysX is pretty much dead by this point anyway and its probably not going to bounce back in the future either.
In times past, when PhysX was used more and when video cards were a lot slower it made some sense to offload some stuff to a 2nd card, but these days the overhead is probably more than the benefit even if you get it working optimally.
See here for a list of PhysX titles old and upcominghttp://en.wikipedia.org/wiki/PhysX
The list is pretty sparse. BF3? Not on there. COD? Not on there. Skyrim? Nope. Amalur: Reckoning? Nope.
Batman, Borderlands, Metro 2033, Unreal Tournament, and a couple other titles are probably all that you will have heard of. The up and coming list isn't hugely populated either.