The iPhone Duo's two-app multitasking feature works side by side in landscape, but rotating the inner display to portrait usually drops you back to a single app. However, there is a workaround to keep a video playing above an open app in portrait, and it involves Picture in Picture.

Both the inner and outer displays on the iPhone Duo share the same 1:1.4 aspect ratio, and each half of the inner display works out to roughly the same size and shape as the outer screen. That even split gives each app a canvas it already has to support, with the divider sitting neatly on the fold.…