i drained my system and installed 2nd lower card in the first slot and nothing ...have you tested all the pci-e power ports on the psu with the with both sets of cards? I'm not really sure how you are figuring this stuff out etc.
The tech specs on corsair website say this PSU has 4 PCIE connectors not 2. After all is a 750W. I had a 650W that was having 4, so it would have been a bit lame for Corsair to showcase only 2, on quite a high powered PSU on single rail design.