A while ago I wrote a blog post called How to Start a Split Screen on Mac which still works on older versions of macOS, but when Apple released macOS Catalina, they changed how it looks a little so here’s how to start a split screen: updated for macOS Catalina!
How to Start a Split Screen on Mac: Updated for macOS Catalina
1) Have at least two apps open at once, then choose an app to start.
2) Hold the Green button in your starting app, then choose a side, left or right.
3) Click the second app.