How do I use both GPUs on a MacPro?
Have you tried doing some heavy graphics load on two different monitors? I've read in the past that the 2013 Mac Pro attempts to use only one GPU for the most part and leave the second one available for OpenCL / GPU compute functions. However, if you increase the load on the GPU enough it will switch one of the displays over to the unused GPU (e.g. run a demo like Unigine Heaven on one and Valley on the other screen).

Note that macOS does NOT support two GPUs driving one display (a.k.a. CrossFire or SLI), so you would only use the second GPU for graphics if you have two displays connected and the load is sufficient to switch on the second GPU.