The answer for product teams
iPhone Duo makes continuity—not a wider canvas—the central design problem. A person may begin a task on the outer display, open the device, compare two views, and return to a compact posture. The app should preserve the task, selection, scroll position, and unsaved work through each transition.
Start with task geometry
Map one high-frequency job before designing layouts. Reading, approving, editing, and monitoring have different needs. A messaging app may keep conversation context on one side and composition on the other; a finance app may reveal detail only when it materially improves a decision. Avoid placing controls on both screens merely because space exists.
Test interruption, not just expansion
Run usability tests that include folding during an upload, moving to background, rotating, and reconnecting after weak network service. Record whether the user can explain what happened. A graceful two-screen layout that loses state is worse than a focused one-screen experience.
FAQ
Do existing iPhone layouts need a separate foldable redesign? Start with adaptable hierarchy and state restoration, then add dual-pane behavior only where it reduces steps. What is the launch risk? Treat device-specific assumptions as provisional until Apple publishes final platform guidance.