Go up to the green expand screen circle in the top left of your browser of one of the windows you want to have it split screen. Hold it down for a few seconds and a blue screen will appear. Move the window to it. Pick another window and move it to the other side the same way. Click the green circle to stop.
You will have to hit Mission Control in your dock and retrieve on the windows from the top of your screen. That is the only drawback but takes an extra second when you get the hang of it.
Official help:
https://support.apple.com/en-us/HT204948