XPS 8920, second GPU not recognized?

Hi,

I have the i7 7700k version of the Dell XPS desktop, which came with the Nvidia GTX 1070. I want to add another GTX 1070 GPU to do scientific computing, but it is not recognized by windows. The second GPU is powered by another PSU, leaving the stock 460w one just for the original system. Is there any restriction on the motherboard for dual GPU? In the BIOS there is no setting to adjust this. Is there anything I should to windows be able to identify and show both GPU's?

Thanks,

Aleg