Issues when using GPU riser cards

[Elie]

Hello,

I am new to Unraid so please bare with me

I have a Dell 3930 rack workstation which has 2 x NVIDIA P4000 GPUs. I have managed to successfully pass the two GPUs to one VM. I also set the ROM path and I am able to get video output from the cards
 

The issue I have is that in windows device manager the 2nd GPU has a yellow explanation mark with a "Windows has stopped this device because it has reported problems". The 2nd GPU doesn't even appear in task manager or in any application I use. Even NVDIA-SMI doesn't show the device.  

Next, I tried to assign only one GPU to the VM and it didn't work. Then I tried to assign the 2nd GPU alone and it still didn't work. This really puzzled me so I opened up the cover of the server and I noticed that there is a riser card slotted into a PCIE slot and both GPUs are connected to the riser card.

I have good reason to believe that this could be the issue. Perhaps UNRAID is seeing only one device on this PCIE slot and has trouble with the 2nd. So is there a way to verify this ? Any settings or workaround that can be applied?
I would really like to have both GPUs operational on one VM or at least get one GPU operational in 2 VMs. I am using Unraid 6.8.3
thanks!