I have a system with two Vega 64 graphic cards but only one is recognized. When I look at AMD Radeon Settings > System I only shows "Primary/Discrete". The power leds on the not recognized card is only one green. The recognized one show lights depending on the load. I have a Corsair 860i PSU which should deliver enought power for both cards. At one point actually both did work.
My OS is Windows 10 Pro.
Actually it comes and goes... when I reboot the system sometimes I see the second card "Linked/Discreet" and it also works. But when rebooting it's randomly not there... any ideas ?