Yes, the 2xhd6990 is faster, but not much:
1x hd7990 @ ntlm : 32986 MH/s
2x hd6990 @ ntlm : 39151 MH/s
But you save the pcie slot and the power consumption
1x hd7990 @ ntlm : 32986 MH/s
2x hd6990 @ ntlm : 39151 MH/s
But you save the pcie slot and the power consumption