How do I use both GPUs on a MacPro? My MacPro (Late 2013, OSX Yosemite) has two AMD FirePro D500 GPUs and three Cinema HD Displays.  When I look at the System report all of the Displays show up under one GPU and the other GPU has nothing.  

Is the other GPU (slot-1) being utilized at all?  How do I tell?  

Can I move a display to the unused GPU?
Will this help performance for rendering or calculations?

AMD FirePro D500:

  Chipset Model:    AMD FirePro D500
  Type: GPU
  Bus:  PCIe
  Slot: Slot-1
  PCIe Lane Width:  x16
  VRAM (Total): 3072 MB
  Vendor:   ATI (0x1002)
  Device ID:    0x679e
  Revision ID:  0x0000
  ROM Revision: 113-C3861J-687
  gMux Version: 4.0.11 [3.2.8]
  EFI Driver Version:   01.0B.687

AMD FirePro D500:

  Chipset Model:    AMD FirePro D500
  Type: GPU
  Bus:  PCIe
  Slot: Slot-2
  PCIe Lane Width:  x16
  VRAM (Total): 3072 MB
  Vendor:   ATI (0x1002)
  Device ID:    0x679e
  Revision ID:  0x0000
  ROM Revision: 113-C3861J-687
  gMux Version: 4.0.11 [3.2.8]
  EFI Driver Version:   01.0B.687
  Displays:
Cinema HD:
  Display Type: LCD
  Resolution:   2560 x 1600
  Pixel Depth:  32-Bit Color (ARGB8888)
  Display Serial Number:    [redacted]
  Main Display: Yes
  Mirror:   Off
  Online:   Yes
  Rotation: Supported
  Adapter Type: Apple Mini DisplayPort To Dual-Link DVI Adapter
  Adapter Firmware Version: 1.03
Cinema HD:
  Display Type: LCD
  Resolution:   2560 x 1600
  Pixel Depth:  32-Bit Color (ARGB8888)
  Display Serial Number:    [redacted]
  Mirror:   Off
  Online:   Yes
  Rotation: Supported
  Adapter Type: Apple Mini DisplayPort To Dual-Link DVI Adapter
  Adapter Firmware Version: 1.03
Cinema HD:
  Display Type: LCD
  Resolution:   2560 x 1600
  Pixel Depth:  32-Bit Color (ARGB8888)
  Display Serial Number:    [redacted]
  Mirror:   Off
  Online:   Yes
  Rotation: Supported
  Adapter Type: Apple Mini DisplayPort To Dual-Link DVI Adapter
  Adapter Firmware Version: 1.01
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.