Video will show what happens but it happened one day when I started up Payday 2 and I got to the menu, clicked on crime net and freeze then BSOD, I keep on restarting my PC but after the windows loading screen it BSOD'd over and over again. I look at the BSOD data and it says the nvidia driver nvlddmkm.sys. So I go into safe mode networking and remove the driver restart, and log back in normally and download and install I think the 329 driver restart and now the screen after the loading windows screen is black for 2-3 min both screens on standby. No HDD activity since the HDD light is blinking every once and awhile. After those 2-3 minutes, one of the GPU fans kick on to 80% or higher as it sounds and during gameplay I can tell only 1 GPU is working, and my FPS isn't always 60. I have 2 GTX 580's btw.
https://www.youtube.com/watch?v=PCi2HGfNrVw
Try each GPU on their own?
Then if they both work individually you'll at least know they aren't the cause. If not then you'll have found a fault.