in "Compatibility" set SLI compatibility bits (DX1x) to Bf3/bf4 which is 0x080116F5
in "Common" set Multi-display/mixed-GPU acceleration to "Single display performance mode" also Power management mode to "Prefer maximum performance" ( I assume you are playing on single display )
in "SLI" set Number of GPUs to use on SLI rendering mode to "SLI_GPU_COUNT_TWO" and under it in NVIDIA predefined number of GPUs to use on SLI rendering m... to "SLI_PREDEFINED_GPU_COUNT_DX10_TWO"
Doing this should get your SLI running and a nice boost in FPS. I figured out how by using search engines with "enable sli battlefield 1" and "battlefield 1 enable sli" so I don't know of anything else that could/should be touched.