I don't think there is a minimum version other than having an Unraid-Nvidia build running. All of the functions I used in PHP are at least 5.x plus and UnRAID is currently 7.x.
That being said, my guess is you're reaching a program error due to the dual video cards in your system if nothing is displaying at all. I will need two things from you to fix this:
1) the output of nvidia-smi -q -x
2) the output of:
cd /usr/local/emhttp/plugins/gpustat
php ./gpustatus.php
It should echo out a JSON string or an error depending upon what's happening. Again, as my first post stated I did not have the ability to test more than one GPU being installed. You can help me blaze the trail with this info!