That motherboard has two physical 16x slots, though electrically the 2nd one is only 4x.
As long as a slot is 8x or better (PCIe2 or 3 doesn't matter), it wont bottleneck a card slotted into it, running a card in the 2nd slot will drag down the performance of the array. My suggestion is you jut buy an outright stronger card instead of crossfiring two weaker cards.