Open the first app you want to use
Tap the Recent Apps button or swipe up and hold, depending on your phone
Find the app in the app switcher
Tap the app icon at the top of the app preview
Select Split screen or Open in split screen view
Choose the second app from the recent apps list or app drawer
Drag the divider up or down to adjust the screen size
To exit split screen, drag the divider all the way to the top or bottom, or close one of the apps