Yes, you have to have both power supply plugs plugged in.
Do note, its a powerful card, and some Power Supplies may not even have 2 video card power connectors.
These are changeable though with adapters as I've just checked, so you could change a standard molex connector to a video card 6 pin connector:
If you do have to use adapters to get the 2 required, you have to make sure that you are using 2x 12v seperate rails, and not just sharing one rail.
I.e, use 2 seperate power cords coming from the power supply, so two seperate connectors, not shared on the same cord, i.e by using a splitter.