This was posted in this forum for similar problem ( it's about bios settings ) :
https://www.youtube.com/watch?v=H-Ttlv9Sg28

Do you have connected both the 2  4-pin power PCIE connectors right?...i think you did but...

If it doesn't work i would reset cmos reinstall OS, modo driver and then add 1 card, reboot, add another and so on...