YES YOU NEED TO USE BOTH 6PIN AS EACH OF THOSE DELIVERS UPTO 75 WATT
GTX 660 TI TDP 150 WATT. IF YOU DONT PLUG THEM PROPERLY YOUR CARD MIGHT NOT RUN. IF YOU DO NOT HAVE 6PIN CONNECTOR YOU CAN PICK UP MOLEX TO PCIE POWER CONNECTOR LIKE THIS
HOPE THIS HELPS