Well before contacting EVGA support, consider that if the upper card runs 10Â°C hotter than the lower card, much probably it's because the lower card does not let enough freash air to be sucked by the upper card heatsink fan. Basically, while the lower card gets fresh air, the upper card gets air that is heated up by the lower card, which does get hot even in the back side. Consider eventually to put a fan to blow fresh air directly on the side of both cards, from the side panel. Of course, without having an idea of your case and components installed in it, it's hard to give a better idea.
Agreed. Back when I ran two cards, the temps were something like this with all solid slot covers in place:
Top card: 85*
Bottom card: 65*
I removed the slot covers (see picture) and the split dropped to only 10 degrees instead of 20 degrees. Like garacs1 said, heat rises, and heat comes off even the back of the PCB, which is something that you can't do much about besides opening up slot covers to try to get the case fans to draw more air in via that path. With the evga, I would hope you could get the temperature split even lower since they exhaust at least half the heat out the back, as opposed to mine which dump all the heat into the case.
If you want to be SURE it is a positional problem and not a physical card problem, that's a piece of cake to test: just reverse the two cards positions. If the top card is STILL the hotter card, then you know there's nothing wrong with the cards, and it's all about the position. Alternately, just remove the bottom card and see what happens to the temps on the top card.