i've prepared recently a mp 3.1 for a client, witha gigabyte 970 video card, 2x dvi, 3x displayport, 1x hdmi, mac efi rom, all tested ok on cmp 5.1 with single cinema display, all video ports has activated for mac systeme and with boot screen, but has issue on mp3.1, my client he has two cinema display having dvi connector, and the mp 3.1 is on el capitan, when he connected two dvi monitors, the system doesn't have boot screen, but enter to the menu/desktop directly, and it was identified correctly by the system, when we disconnect one dvi monitor, the system start with boot screen, and of course your can combine with other connectors: hdmi and displayport, all works with one single dvi, dont know why not with two, havn't testes on mp5.1 with two dvi, dont know if it acts the same way. hope this will help when met the same condition.