Hi,
Take a look at this user's solution to the split screen problem:
https://discussions.apple.com/thread/8592364
As per the above thread, click on your project's name under Project Media in the upper left hand corner of your project screen. The clips in your project will display in the Media Browser screen to the left of the preview screen. Select one of the clips that you want to use in your split screen. It will display in the preview screen. Then click on the crop tool (the one that looks like intersecting angles) in the tool bar at the upper right of your screen. A little rectangular "Crop" button will appear above and to the left of the preview screen. Click on that. Adjust the rectangles that appear in the preview screen to crop the part that you want visible. Do the same with the other clip. Then drag them into the timeline and do your split screen. You might need to fiddle with it a little.
-- Rich