Select specific gpu combo

Hello, so i have a pc with an nvidia quadro k620 and a gtx 590 which is known for being 2 card in one. Now when I enable the gpu for cycles i can only select one 590, the other one, the k620 or 590+590+k620, so of course my question is: how do I enable just the combination of the 590 (590+590) and leave the k620 to the viewport?

Thanks in advance
:o