I have 2 GPUs, but my NVIDIA-SMI just shows me one. How can I make it recognize the other one?
I think GPU don't appears if is disabled. Check if "remove" file in your GPU device is empty. In your case, see next file; /sys/bus/pci/devices/0000:83:00.0/remove
If this folder doesn't exists your device hasn't started correctly. In this case, If is possible for you, try checking that this GPU have enough power and is correctly connected.
If there is 1 in the file, change it by a 0 and reboot. If the folder exists but the file don't contains the value of 1 You can check other files in this folder for more information about the device because your device starts and some other things are causing the faulure. Check using other older driver version as says vfbsilva.
Font: I disabled some GPUs manually and the results are similar. https://labernia.wordpress.com/2017/07/16/gpu-testing/