The iPhone Duo can run two apps side by side in landscape, but rotating the inner display to portrait typically returns you to a single app. There is a workaround, however: by using Picture in Picture you can keep a video playing above an open app while the device is unfolded in portrait.
Both the inner and outer displays on the iPhone Duo share a 1:1.4 aspect ratio, and each half of the inner display is roughly the same size and shape as the outer screen. That split gives each app a canvas it already supports, with the divider aligned on the fold; the notable exception appears when you try to watch a TV show or movie above another app in portrait — the following trick addresses that.
iPhone Duo Picture in Picture workaround
Start playback in an app that supports Picture in Picture, for example Apple TV or Apple Podcasts. Tap the Picture in Picture icon to shrink the video into a floating window. Swipe up to return to the Home Screen, then open the app you want to use (it cannot be another video app). Finally, tap the icon that appears in the PiP window to stack the video above the open app.
The method is somewhat fiddly but works for most apps that support PiP. Whether Apple will make this behaviour easier in a future update remains to be seen.
