Split Screen mode in macOS has been around for some time and it’s a great way to view two documents at once, but in a fullscreen mode. This even works with two applications, as long as the windows are in the same Space.
Click and hold the green fullscreen button at the upper left corner of the window. You can then move a window to one side of the screen to use it in Split Screen. Then select the next window and you’ll have both open, side by side. However, you may find that Split Screen isn’t working. Here’s a fix for that: Go into System Preferences > Mission Control and check the checkbox for Displays have separate Spaces. Then log out and Split Screen should work after you log back in.