Thanks for the guide, was very helpful. Can you control fan speed of 2 or more cards?
For some reason, I can only control fans of one card, but not both (I have a 980 Ti and 970 installed, 980 Ti is adjustable). Works flawless otherwise.
Assuming your fan controls are built to Nvidia spec, this activates both card controls:
sudo nvidia-xconfig -a --cool-bits=28 --allow-empty-initial-configuration
Doing it via the Nvidia X Server Settings
GPU 0 or GPU 1
Thermal Settings, move slider to the desired fan speed and click apply.
Or, you can create sh profile and attach it to the startup as the guide explains.