Multiple keyboards do not play well with my BIOS

Currently I'm at a LAN party and this morning I connected my Razer Tartarus to my PC. When I rebooted, suddenly I was unable to select any GRUB entry (my keyboard didn't work).

Razer Tartarus - picture from Razer

It worked fine yesterday and this morning, so what happened? My gut told me to try to use my Tartarus (which basically is a keyboard) and voila, that worked. Then I connected the Tartarus to my non-onboard USB ports and my Das Keyboard Model S Ultimate to an onboard USB port and voila, the keyboard priority changed back to my normal keyboard again.

Das Keyboard Model S Ultimate - picture from Das Keyboard

Strange that the BIOS (UEFI) only can handle 1 keyboard at a time... Maybe it is a limitation of my Asrock Z97M OC Formula, who cares. Just glad I debugged this so quickly.