Hey,
I have a similar setup like jammsen.
Asus Prime B350 Plus
AMD R7 1800X
16GB DDR4 RAM
NVIDIA Gainward GTX 1080 (Primary)
NVIDIA MSI GTX 1050TI LP (Secondary)
I first encounter the same Problem like jammsen, the IOMMU Group were far from optimal.
Thanks to david279 the multifunction part did the trick and i was able to assign the secondary graphics Card aswell.
However, after passing trough the first one I got another error in my VM log:
"qemu-system-x86_64: vfio_region_write(0000:01:00.0:region1+0x240c14, 0xffffffff,4) failed: Device or resource busy"
From my Point of view, this indicates, that the host is grabbing the graphic Card resources itself and therefore can't passtrough the device.
After some research found a solution in the redhat Mailing List:
https://www.redhat.com/archives/vfio-users/2016-March/msg00083.html
After executing the commands above unraid was able to passtrough the first device aswell.
Downside of this solution is, that I need to to the commands after each host restart.
In the comining days i will try to find a permanent solution.