Are both D500 graphics cards functioning?

Your Mac Pro has all six display interfaces connected to one of the G300, G500, or G700 cards by design.

The first card does not need any help with rendering ordinary screen real-estate, even for six large displays. Most of the time it is not working very hard. The second card will only become active when needed, and is generally reserved for GPU computation and complex off-screen rendering.

The reason fot this design is that interrupting GPU computation is incredibly destructive. Interrupting an intensive high-powered computation for mundane screen update is wasteful and completely unnecessary. The second GPU is free to do its computations and rendering without any interruptions.

when used in this way, the two GPUs are Much more powerful.

Hi, Rob_D !

The only way to tell if your MacPro is functioning would be to take it to an Apple Store or Apple Authorized Service Provider (AASP) for a diagnostic.

That being said, the D500s are incredibly powerful video cards even by today's standards. I had a D500 MacPro with four 23" HDMI monitors, and it had yet to even start breathing hard. When the power is needed, your MacPro will access it. If that power is not needed, the MacPro will not access it. I have no idea why you would want to 'force' the MacPro to do what it does not need to do. Give it the reins and let it run. Don't go mucking about and make a mess of it.

I hope this helps, and good luck !

not the best answer but interesting. Seems its up to third party developers.

https://www.anandtech.com/show/7603/mac-pro-review-late-2013/9