Dual GPU (2x AMD RX460) black screen issue

[Naruto01]

I recently got a second RX460 for cheap. Now I want to use it with my existing RX460 (the cards are identical) on 10.12.4 in a dual GPU setup. The single RX460 (Lilu, WhateverGreen) runs just fine, but as soon as plug in the second one in the x8 lane slot the system boots to a black screen. I can ssh into that machine, so it's running except the black screen. I also own a 750 GTX (Webdrivers, Lilu, Nvidiagraphicsfixup) which runs very well on the x8 slot, so it must be related to the second RX460. I even flashed the second RX460 to a RX560 and after that the black screen was gone but the card wasn't properly recognized https://showbox.bio/ https://tutuapp.uno/.

Motherboard: Gigabyte Z170MX Gaming 5
CPU: Pentium G4560
GPU: 2x Gigabyte AMD RX460

$ system_profiler | grep -A35 Graphics/Displays

Graphics/Displays:

Radeon Pro 460:

  Chipset Model: Radeon Pro 460
  Type: GPU
  Bus: PCIe
  PCIe Lane Width: x8
  VRAM (Total): 4096 MB
  Vendor: ATI (0x1002)
  Device ID: 0x67ef
  Revision ID: 0x00cf
  EFI Driver Version: WEAR-104-2017-08-27
  Metal: Supported
  Displays:

Hardware:

Hardware Overview:

  Model Name: iMac
  Model Identifier: iMac17,1
  Processor Name: Intel Core i5
  Processor Speed: 3,50 GHz
  Number of Processors: 1
  Total Number of Cores: 2

What could be the reason? How do I get the two cards to work? 

Edited April 6, 2021 by Naruto01