I just bought another 390x. Tested and it works alone.
I have 2x MSI R9 390X OC, same BIOS version.
I can't for the life of me to get crossfire to run. I'm running 18.9.3 and I have crossfire enabled in Radeon Global Settings. It sees both cards.
MSI Afterburner doesn't show any usage, fan, and holds a constant 35C temp for the second card...shows activity for the original card. Tested with Rise of the Tomb Raider and nothing, I've tried something like 3DMark and no usage. Windows Task Manager doesn't see any usage but shows it there. It shows up in Device Manager as okay.
I have a Gigabyte Z97X-Gaming 3 motherboard with F6 BIOS. Windows 10, up to date.
I found something on another site to check on "Disable ULPS" within MSI Afterburner and reboot...same results (still checked on).
Setting a specific profile within the Radeon Settings for say, Rise of the Tomb Raider (AMD profile) does nothing. I don't know what I'm missing and it's annoying...lol