On the Galaxy S5 you could hold the back arrow down and it would pop up the multi window on the left hand side. I can not find how to activate. Thanks!
Drag diagonally from the top of the screen, or use the multi-task screen and press the two square button on the app title bar. Keep in mind not every app supports multiple window
If you open up the app switcher you should see a small double square symbol to the left of the X on a app that can do multi windows and that is how you start it up.