I want to say I seen it done the other way, with NVidia as the primary. Its been a long time since I have seen it tho, but want
to say if I recall correctly, and I may be wrong, but it had something to do with the NVidia card only loading as a physics card if it
was secondary. I don't see why you cant mix and match them tho.
Best way to find out tho, would be trial and error.