It worked for me, with two nVidia 1080s running in SLI and using NVidia's surround screen. It might work for other configurations as well.
Switch to openGL rendering mode.
Run OBS with the --allow-openGL switch to enable the setting. Only needs to be done once, the setting stays on openGL on subsequent runnings.
Switch to openGL rendering mode.
Run OBS with the --allow-openGL switch to enable the setting. Only needs to be done once, the setting stays on openGL on subsequent runnings.