How to Split Screen on Chromebook

Open your first app and click the window size reducer button in the top-right corner of the window.

Arrow

Drag the window to either side of the screen 

Open your second app and click the window size-reducer button in the top-right corner.

Drag the window into the other side of the screen and release when you see that side of the screen gray out.

You can always go back to the typical window configuration by clicking the window-expand button in the top-right corner of either app window.