SLI does not work in this game, and Remedy all but said it would never be supported.
Quantum Break - State of the Game UPDATED 28.4.2016 - Remedy Community Forums
"Quantum Break on Windows 10 unfortunately doesn’t support DirectX12 multi-GPU rendering. Due to the engine architecture, the work needed to support multiple GPUs would have been significant and out of scope for Remedy for Quantum Break."
Quantum Break - State of the Game UPDATED 28.4.2016 - Remedy Community Forums
"Quantum Break on Windows 10 unfortunately doesn’t support DirectX12 multi-GPU rendering. Due to the engine architecture, the work needed to support multiple GPUs would have been significant and out of scope for Remedy for Quantum Break."

