The 'Split screen' feature allows multiple apps to be run on the current screen at the same time, is turned on by default and can't be turned off.
From any screen, tap the Recent apps icon (lower-right).
Scroll left or right to locate the preferred app then tap the app icon (e.g., Calendar, Gallery, Email, etc.) located at the top of the panel.
Tap Split screen.
From the Recent apps screen, tap the second application to be viewed.
When on, tap the Recent apps icon (lower-right) to change the current app.
Adjust the size of the apps by sliding the horizontal bar between the applications up or down.
To leave split screen view, slide the horizontal bar all the way to the top or bottom.