Open the first app
Swipe up from the bottom to show the Dock
Touch and hold the second app in the Dock
Drag the second app to the left or right edge of the screen
Release it when the screen splits
Adjust the divider to change each app’s size
To switch an app to Slide Over, drag it to the middle of the screen
To close split screen, drag the divider all the way to one side
If needed, turn on multitasking in Settings > Home Screen & Multitasking > Multitasking