If the choice is either two 285's in SLi doing gfx and PhysX or a single 285 doing gfx and another doing PhysX then I'd say the the SLi option would be better, probably by a long way.
PhysX doesn't require that
much power, so a 285 purely for PhysX is pretty major overkill. You would have much more benefit using both in SLi and letting PhysX take whatever power it needs to do it's thing. Basically what you're asking is; will the demands of PhysX make two cards in SLi running gfx and PhysX slower than a single card just running gfx? No, not by a long shot
Of course SLi + PhysX (3 cards) with something like a 9600/9800GT dedicated to PhysX is the best way to go, but a lack of room on the mobo is an issue for many.
Another thing to think about is just altering the settings for the rare occasion you play a PhysX enabled game.
Hope that made sense