You can't set PhysX to run on both cards, it's either one or the other.
Sorry, I don't do SLI yet. I just want to be clear, that when you have SLI and want to do PhysX, are you saying one of the cards does both graphics and PhysX and one does just graphics? Are you saying it's impossible to have both cards in SLI handle both graphics and PhysX. The last thing we want to do in a proper SLI setup is dedicate one of the cards to PhysX, while the other handles all the graphics by itself.