Sounds like a card problem if 1 fan is spinning. These newer GPUs(certain models)are made in a way that at low temps, the fans are off completely. However, as temps rise both fans should turn on. A quick search shows others having the same problem which ended up getting an RMA replacement. Others have simply plugged GPU in another slot or changed drivers and got both spinning, though I'm not really convinced this is a fix, so much as a flaky fan.