Which motherboard do you use? For some mothboards, if you use the x1 riser, you need to short the A1B17 or B1A17 card presence pin.
MSI P67a-GD65
where is that pin located?
Shorting out the pins just tells the motherboard to provide 75W to the slot, usually x1 is limited to 25W.
It may result in more power going through the MB than it can handle. I don't recommend it.
Make sure you use powered risers. With powered risers the power from the slot is not used. My rule of thumb
is the MB is designed to supply enough power to as many GPUs as there are x16 slots. Any more GPUs than
that and extra power is needed.