I have 2 radeon hd 6970s running in crossfire mode. They are both overclocked and have been running perfect for weeks. All of the sudden ( and no not after a driver update), The first of two starts getting hot ( 67c at idle), meanwhile the other is idling at a chilly 32c. Up until today when this happened it had been running at similar temps to the second one. I replaced the thermal paste about a month or so ago so it should be fine on that. What's more is that now I can't toggle the manual fan control, if I move the slider to 100 it "applies it" but no more fan noise, no loud whirring,nothing.
The fan is definitely still going( maybe 30 percent and even with that it shouldn't be at 67c ) but not reacting to the fan controller at all.
Probably FAN controller of your second card is faulty. You need to contact any repair shop or maybe buy some non reference cooling system.
Also you can try to leave only this card in your PC for experiment (and install it in another PCIe slot too)