Kicker,
This should not make any difference in your score, RealBench uses all your GPU's in the OpenCL portion of this benchmark no matter the order, you can have Nvida and AMD and even iGPU's running at the same time.
Disable the 680 using the pcie lane switch, then run the benchmark, then re-enable it and run the benchmark again to compare your scores, you can check this faster by only running the OpenCL test.