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.