Open the two apps you want to use
Click and hold the green full-screen button in the top-left corner of one app window
Choose Tile Window to Left of Screen or Tile Window to Right of Screen
Select the other app from the other side of the screen
Drag the divider in the middle to adjust the split
To exit split screen, move the pointer to the top of the screen and click the green button again
You can also press Mission Control and drag a window onto another full-screen app to create split view