Open the two apps you want to use
Hover over the green full-screen button in the top-left corner of one window
Click and hold the green button
Choose Tile Window to Left of Screen or Tile Window to Right of Screen
Select the other app or window for the opposite side
Drag the divider in the middle to resize the two windows
To exit split screen, move the pointer to the top of the screen
Click the green full-screen button again on either window