Problem resolving similarly named devices

Hey all, I am having an issue with multiple devices of the same type. I've seen it pop up a couple of times.

The issue is if I have two devices of the same type, such as two similar graphics cards. I want to assign each to a seperate VM, but can't do so. I have two AMD Radeon 7950 graphics cards, and I can assign one to a VM (let's call it WORK), and it works fine. The second graphics card is assigned to another VM (let's call that one KATH), and is connected to a seperate monitor. I've assigned the first card on the GPU drop-down to WORK, and it boots fine. But when I assign the second GPU on the drop down to KATH, it fails to start with an error of this:

internal error: early end of file from monitor: possible problem:
2016-01-21T17:17:42.824271Z qemu-system-x86_64: -device vfio-pci,host=83:00.0,bus=root.1,addr=00.0,multifunction=on,x-vga=on: vfio: error opening /dev/vfio/43: Operation not permitted
2016-01-21T17:17:42.824310Z qemu-system-x86_64: -device vfio-pci,host=83:00.0,bus=root.1,addr=00.0,multifunction=on,x-vga=on: vfio: failed to get group 43
2016-01-21T17:17:42.824326Z qemu-system-x86_64: -device vfio-pci,host=83:00.0,bus=root.1,addr=00.0,multifunction=on,x-vga=on: Device initialization failed
2016-01-21T17:17:42.824340Z qemu-system-x86_64: -device vfio-pci,host=83:00.0,bus=root.1,addr=00.0,multifunction=on,x-vga=on: Device 'vfio-pci' could not be initialized

I am assuming that what is happening here is that the system is getting confused about GPUs and is trying to assign the first card to KATH, only to find that it is being used by WORK.

I had a similar problem come up when I tried two identical USB hard drive controllers (both made by Samsung): the system wasn't able to correctly resolve which was which.

Any ideas? I could pull the 2nd GFX card and use another one, but I would like to use this one if possible.

Ta,

Richard