In sli the top card will always run hotter then the lower one. The top card gets less air flow and raising heat off the lower one.
There are work arounds for games that dont like sli like forcing the cards to alternate every frame. top card will do all the odd frames and the lower card will do all the even frames. Back when The elder scrolls online came out i had to force my two 580's to do that other wise the lower card was not used.