SLi or tri-SLi (Crossfire for that matter too) is always going to present heat problems for the top card. Many folks claim that for multi-gpu configurations that the stock squirrel cage fans actually do a better job of getting heat out of the case than the custom cooling solutions found on most high end cards.
I think you have a few options ranging from free to expensive;
Place a 120mm or 140mm fan in the side panel of your case blowing directly between cards #1 and #2
Create a custom fan profile and see if running the gpu cooler fan at higher speed reduces the temperature
Use your third party o/c utility of choice to reduce the gpu clock frequency on one or more of the cards
Last but not least, in my experience it has always helped from a little to a lot to re-do the factory thermal paste application. Take care not to damage the thermal pads between the heatsink base and the memory / vrms. You'll want to re-use those if you're not planning on replacing.
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