Tap the Dual window switch (at the upper-right corner of the screen) to toggle it On or Off. On enables you to touch and hold the Back Key (in the Touch Keys bar) to activate two app windows at the same time.
Split view: Checkmark to automatically open the Dual window feature when you tap a link on the full screen or tap an email attachment (images only).
Help: Displays help information about using the Dual window feature.