Does Just a Rock work on iPhone Duo?

Not yet. Just a Rock runs letterboxed on the inner display, with black borders around an iPhone-sized window. Its latest version is built with the iOS 26.2 SDK, which Apple shows letterboxed on the inner display.

Is Just a Rock letterboxed on iPhone Duo?

Apps built with the iOS 26 SDK or earlier show black borders around an iPhone-sized window on the inner display; iOS 27 apps resize to fill most of it, and iOS 27.1 apps fill it completely. Current status: Letterboxed.