Originally Posted by methimpikehoses
I switched the cards, and the problem seems to be corrected. Played a round of 2k10 and the fans stayed at a normal quiet speed. We'll see how it goes.
IIRC when you have to unmatched cards the primary card will control the clock speed for both. So it might be possible that the 2nd card was overclocked to match your first card, causing it to not work right. (Basically the same thing bacon said)