From the Home screen > Settings > My Device. Then, go to Display > Check Multi Window:
From the idle screen, press and hold the Back button to display the Multi-View drawer. It contains a list of apps that can be launch in Multi-View Mode. Click on an application and drag it to the phone screen:
Click and drag another application to the upper or lower half of the screen:
Now you have two applications running on the screen. You can use the bar that divides the two applications to resize the size of each window.
Note: Not all applications are compatible with Multi Window.
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