The most likely answer is that the 1st card would not be getting as much airflow, as the 2nd (bottom) one to cool it. I have a friend with the same thing and those zalmans are big, the fan nearly reaches to the bottom card... right? Much of the cooling potential of the zalman is due to the numerous copper fins, but "straight down" or in this case "straight up" cooling onto a chip is not the most efficient way of cooling, it creates a deadspot right over the chip, not as good as... say... side cooling. That being said, the zalman is a fine, quiet, very good cooler, better than just about any stock cooler, it just would be even better (though probably louder) if it was a side cooler. It's why I also prefer the side cooling cpu fans, like zalmans and the arctic cooler freezer 64 (which I own). Though, if space is a concern they are much much taller.
This is probably the biggest reason, because, also you are drawing hot air from the other side of the card below (the "top" of these cards can get damn hot thru the pcb) and are drawing it right onto your other gpu.
Also, the probablility that one card is doing more work than the other is quite high, for example if you forgot to enable SLI mode through the driver itself and not just the motherboard.
Also, not every game uses SLI in the same as the next, meaning they don't always, if at all, spread the love between the two cards.
You can figure this out, by just removing the 2nd card, or disabling the SLI in software thru the nVidia driver again, and see the performance difference.
Finally, long ramble short, assuming that you are getting a good performance increase with SLI (i.e. you've been able to prove it's actually working) your temp in the "hotter" card is well within spec and I wouldn't worry about it.