Do you have adequate PSU, anything less than real 650W is too weak for 2 X 7970, even more, depends partially on the rest of hardware. For example, I had almost no problem to put 6770 and 7770 on the same mb with full hash rate of both cards, 550W psu, old 939 mb and processor, dedicated for mining only. So I guess combo with same cards should be even easier to manage. Good luck!