August 2018
The easiest way to enter Multi Window mode is by long-pressing the Recents key while within an app. But to do that, you'll have to first enable the feature from the settings:
Tap Multi window.
Select the view you'd like to use with Multi Window.
And now you can launch Multi Window mode.
Now select the second app by either browsing from the list of running apps or using the More apps button.
Enjoy