Samsung Galaxy S9 / S9+ - How to Use Split Screen / Multi window
Multi-window is also referred to as split screen.
The Multi window feature allows multiple apps to be run on the current screen at the same time.
From a Home screen, tap the Recent apps icon (bottom-left).
Scroll left or right to locate the preferred app then tap the app icon (e.g., calendar, gallery, email) located at the top of the panel.
Tap Open in split screen view.
From the Recent apps screen tap the second application to be viewed.
When on, tap the Recent apps icon (lower-left) to change the current app.
Adjust the size of the apps by sliding the horizontal bar between the apps up or down.