I have had several problems since my phone updated to nougat. One being the split screen mode (I think). Now I often see where the little square box is at right bottom of screen two smaller rectangles and when I press it there are two things displayed. I have the text size increased as my vision is not good enough to see small text so the split screen does not have enough info for this feature to be of use. When I do a bing search there seems only to be articles on how to turn on and use split screen, not turn it off. I did not consciously turn it on.
1. Press and hold the split screen icon in the bottom right hand corner of your screen.
2. That's pretty much it.
NOTE: On some devices this doesn't seem to work, almost as if it was turned off. I can't say if that is so on yours or not.
Thanks, that worked on my Moto G4+ to disable the current split screen but does not prevent the split screen from happening again. I don't know what I do to get the split screen to start with, but at least this procedure is a quick and easy way to get rid of it. I could not find in settings how to turn the feature(?) off completely. To me its a nuisance.