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