My suggested steps:-
1) Assign the Physx function to Card 1.
2) Run some gaming benchmarks (eg Mafia 2, Metro 2033, Batman). Note temperature for both cards.
3) Rerun those gaming benchmarks with the Physx function re-assigned to Card 2. Note temperature for both cards.
4) Then, assign Physx function to whichever the card which is
cooler.
Usually (but not always), the top card of a SLI setup gets hotter because the gap between its intake fan and the next card is fairly restricted. So, it is generally the lower card that is cooler.
Now,
here is a tricky part:-
when using a temperature monitor utility (eg Afterburner) to check temperature for both cards (GPU1 and GPU2), make sure you know which refers to the top card and which refers to the bottom card. It is
not always true that GPU1 refers to the top card. Sometimes (and often), GPU1 refers to the bottom card.
Simply put, do not mix up the temp reading between the cards.
For an example, Nvidia Control panel shows that my top card is GPU2 while my bottom card is GPU1.
