Split Screen: Open & Use Split Screen. To open Split Screen, swipe up and hold from the bottom of the screen, select the App icon on the desired app, then select "Multi-Window". The selected app will fill the top half of your screen while the bottom allows you to select another app. Select the desired app. Edit Split Screen. When using Split Screen the windows separation bar will appear in the center of the screen, drag the bar up or down to adjust window size.
To open a new app in Split Screen, swipe up and hold from the bottom of the screen, then swipe to and select the desired application. To close Split Screen, select and drag the separation bar to the top or bottom of the screen. ♪AT&T jingle♪.
Source : AT&T Customer Care