Here's how to use two apps simultaneously on your Galaxy S9 / S9+.
'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.
Not all apps support the 'Multi window' feature.
From a Home screen, tap the
Recent apps icon

(bottom-right).
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-right) to change the current app(s).
Adjust the size of the apps by sliding the blue horizontal bar between the apps up or down.
To exit the split screen view, tap the
Recent apps icon

then tap the
Close icon

(top).