It is hard to get dual AMD cards to work, my Haswell pentium supports 2 RX 480s using IGPU (Framebuffer only no acceleration) the Polaris cards work well in some open CL apps, a slight reduction in open GL performance but still working.
However, if using Braodwell, Skylake or KabyLake systems it is very hard if not impossible to get dual AMD Polaris cards to work. There is one person I know of who has disabled most of his IGPU drivers on a Skylake and has partial dual card support but like my setup with the Haswell pentium.
If you really want to get RX 480 working you may need to get a cheap Gt210 or other Nvidia card as helper or simulated iGPU. $30 US for a cheap helper card, I have found the Nvidia cards work great for this purpose and usually allow full sleep wake function for the RX.
I think you will have better luck running dual Vegas than a combo of Polaris and Vega. It may work with your Skylake or Kaby setup if you disable the IGPU drivers.
Keeping my hopes up. I can have them boot, but no acceleration if I fake a random ID. It might be something missing in Clover.
Sierra 12.6 and High Sierra dont require any fixes for RX cards except for the R9 xxx name. As long as you have a Symbios iMac 14.2-18.1 or Mac Pro 4.1 + they wget full acceleration and 36 CUs without any need to alter kexts.
Nvidia card in slot 0 Rx in Slot 1 for a X99 type setup usually works. Here's my Risen 7 1700 Rx 480 using Nvidia Gt210 as IGPU or Helper card.
I'm still struggling getting RX580 working with Vega as IGPU for an X99 platform on High Sierra. I tried injecting different FB through the clover startup menu, specifically for the RX580 without any luck. Is there anything needed on the clover config for IGPU on x99? As I'm troubleshooting, I was thinking about PCI-E assignment for MacOS.
On my standard setup, I have the main GPU on the 16/X8_1 slot, and the secondary (RX580) on the 16/X8_3 slot. On this configuration it boots to black screen. No changes even if apply different FB names (Guariba, Orinoco, Dayman). When I look at the IOREG output, the VEGA card is on slot BR3A@3
When I swap cards and install the main GPU on the 16/X8_3 slot, and the secondary (RX580) on the 16/X8_1 slot. I get reboots @ gioscreenlockstate. No changes even if apply different FB names (Guariba, Orinoco, Dayman). When I look at the IOREG output, the VEGA card is on slot BR2A@2
I figured that because the OS sees 16/X8_3 slot as BR2A@2, maybe the VEGA needs to be in this slot? How can I get it to stop rebooting?