I got the board yesterday and can't get more than one to work. Have also tried the latest BIOS and Beta BIOS with 4G decoding etc. but nothing works.

I also made your IOMMU settings, even without success.

Please ask for help:)

Use DDU to unistall the Vega drivers.  Leave them installed in the motherboard.  Then try reinstalling.

I actually prefer the next to last BIOS.  There are issues with the latest X350 Gaming Carbon Pro/X350 Gaming Pro bios.