Open the two apps you want to use
Hover over the green full-screen button in the top-left corner of one window
Click Tile Window to Left of Screen or Tile Window to Right of Screen
Select the second app or window for the other side
Adjust the divider in the middle to resize the windows
To exit split screen, move the pointer to the top of the screen and click the green button again
If Split View is not available, use Mission Control and drag windows side by side manually