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?