How to set a wallpaper on the iPhone Duo
iPhone Duo uses one wallpaper for both displays, so the trick is a wide image whose centre also works on its own.
Step 1
Download the wallpaper
Download the wallpaper (or tap “Save wallpaper” in the app). It's one wide image that covers both screens.
Step 2
Choose Use as Wallpaper
Open the image in Photos, tap Share, then choose Use as Wallpaper.
Step 3
Keep the scene centred
The inner display shows the whole image, and the outer display shows the middle of it.
Step 4
Add and set
Tap Add, then choose Set as Wallpaper Pair or customise the Home Screen.