Activating the multi-window/split screen functionality on your Note 9 is simple.
Tap on the Settings icon.
Under Device, look for the Multi-Window option.
A toggle switch will show up at the top right corner; drag the toggle to On.
If you would like to view apps in Multi Window Mode by default, check the box beside the option Open in multi-window view.
Once you’ve activated the feature, you’ll see a half circle symbol on your Galaxy Note 9 screen, which means that you have enabled the Split Screen Mode on your Samsung Galaxy Note 9. You can tap the half circle and the multi-window will be placed at the top of your device screen, and you can then move whatever app you want to use into the multi-window screen. You can also adjust the size of the window to make it easier for you to view