It is not a BIOS restriction. Jeez, even gamers run 2 cards......
I think it is more likely a BIOS setting issue, Biostar for instance require you set a "mining mode" switch in the BIOS.
With no experience on (although have a 50% complete build on the Asrock H110) specific boards it is hard to say, but don't be too deterred, I am 99.9% sure its not the cards that are the issue.
Are you using 390.65? The install on multiple cards can take a while, when you add a 2nd does it show as "Basic Display" or "VGA controller?" or not at all?
Gamers link the cards in SLI, i'm using risers, this is different.
The OS (win or linux) sees only 1 GPU, 2nd one is not there ... however ... if i plug-in the power adapter only into 8 pins connector on GPU (leaving the 6 pins connector empty) the OS sees all GPU just that they are unusable.
Ill keep trying, thanks