Two web pages on one screen

Hello,
Can someone tell me how I did this please. I had two web sites open at the same time. I always minimize one and switch back and forth which can be a pain. I accidently clicked something either on my mouse or keypad, I don't know,by doing this it caused both websites to come up side by side in half pages. What is the name for this and how can I recreate this on purpose?

Signed,
Novice