Oooh... that's a good one.
I guess a lot depends on what resolution you play at. At 1680x1050, my single GTX260 could play Crysis all at Very High, with no AA. Framerate never dipped below 25, which for me was playable.
So if you have a GTX285, I'm sure running them in SLi while have PhysX enabled wont hurt. Only real way to find out is to test each, of course. But I'd just rock SLi and leave PhysX enabled. What *should* happen is the card with the least work load takes over PhysX effects. Should have no impact on framerate.
Good question though.