It's in ADVANCED MODE -> EXTREME TWEAKER -> TWEAKERS' PARADISE -> PCI TUNING = INTERNAL GRAPHICS FROM [AUTO] TO [DISABLE] (That's page 3-18 at the very top of the maximus VI Hero user guide)
Also this site helped me tremendously : http://rog.asus.com/254052013/maximus-motherboards/recommended-settings-for-overclocking-maximus-vi-motherboards/
Also DON'T USE the 8 pin to 8 pin to connect to the card. Use two 6 pin PCI-e to 8 pin connectors. Just fill in the 8 pin connector with the 6 pin + 2 little connector built into the cable or just try the 6 pins by themselves. The other 6 pin connects fine. How does that look?