WyomingKnott :
What happens if you try to run with one card, the one that only gets flashing lights?
Both cards work fine as long as I don't connect them together.
The white flashing LED at the pin connectors indicate that there is not enough power.
2x250w should draw ~ 500 watts from 740w PSU and it should still have 240w to spare or at least 200w....
It's a modular PSU with 2x VGA ports that have 2x (6+2pin) each just what 1080tis need....
Maybe it's worth mentioning that the PSU is 6 years old.
UPDATE: Fixed by using another 850watt PSU