Ultrawide monitors with PBP (Picture by Picture) are becoming more common these days. It allows you to split the screen and connect multiple sources to your monitor (e.g. 2 computers at the same time)
The problem is that the resolution is typically not supported out of the box (E.g. a 2560x1080 monitor, when split into 2 will require a 1280x1080 resolution, which is not standard in Mac OS as of Sierra)
First, we have to reboot in rootless mode, and disable the Integrity Protection, so we can create/override the screen file settings: