Designing Apps for iPhone Duo: Lessons From Adapting Three Apps

By Stephen Dixon

iPhone Duo launched last week, and it's the biggest change to how we hold an iPhone in years. Over the past month I adapted three of my apps for it: Naturalis, AteIQ and Cascade. Each one ended up using the fold in a completely different way.

This post is what I learned along the way. All of it was built and tested in the Xcode 27.1 simulator and on iPad ahead of launch. My own Duo arrives in November, so I'll follow up with anything the real hardware changes.

Design for posture, not a bigger screen

It's tempting to treat an unfolded iPhone Duo as a small iPad. That's the wrong starting point.

The better question is: how is the person holding it right now? Closed in one hand. Open like a book. Laid flat on a table. Half folded and propped up. Each posture suggests a different app.

The rule I ended up with for all three apps: respond to the folds people were going to make anyway, and never demand folds they weren't.

I briefly explored using the hinge as a control, folding to trigger something in the app. I dropped it. Hinges wear, and an app that asks you to keep folding your phone is asking for more than it gives.

Naturalis: a field journal that opens like a book

Naturalis is an illustrated field journal. You photograph a species, and it becomes an entry in your own natural history.

Closed, nothing changes. When you find a new species, the full reveal plays straight away, exactly as it does on any iPhone. Opening the phone is a bonus, never a requirement: open it and the reveal becomes a two-page spread.

For the open layout I went with a 50/50 split, with the crease acting as the gutter of a book. I tried the uneven sidebar split you get from NavigationSplitView, but for a field guide, two equal pages felt right. Your collection on the left, the entry on the right.

A couple of smaller decisions:

  • No fake binding at the crease. No stitching, no shadow. The hardware already draws the spine.
  • The split only appears when both size classes are regular. A regular iPhone in landscape keeps the usual push and pop navigation, so nothing gets cramped.
  • Move the camera shutter off the center. A centered shutter at the bottom sits right on the fold when the phone is open. On the unfolded layout, the shutter is biased to one side.

Naturalis is on the App Store for iPhone.

AteIQ: list on one page, detail on the other

AteIQ is a meal tracker: describe what you ate and get calories and macros back. Most of the app is one focused task, so I kept the Duo work targeted rather than rebuilding everything.

Two places got the two-page treatment:

  • Meal History. On the unfolded screen, the calendar becomes a sidebar and the selected day fills the other page. On a regular iPhone it's an Apple Calendar style week strip that pulls down into a month view.
  • Settings. These follow Apple's own Settings app: the list on the left page, the selected item on the right.

I decided against turning the whole app into a sidebar layout. Two-page layouts are great for browsing. They're not automatically better for doing one thing quickly.

Keeping content clear of the fold is handled with the new reservedRegions(.division) API in iOS 27.1, which tells the layout where the seam is so nothing important lands on it.

AteIQ is on the App Store for iPhone.

Cascade: the table between two players

Cascade is a two-player card game my son invented. This is the app where the fold matters most.

Unfold iPhone Duo and lay it flat on the table between you. Each player gets their own side, turned to face them, with the shared cards in the middle. Your hand rests face down on your side, and you hold a card to peek at it.

The bug worth sharing: on a single phone, pass and play swaps the board every turn, because the phone is handed across. On the unfolded layout that's exactly wrong. Each player's side has to stay theirs, so the two pages must never swap.

Without Duo hardware, iPad was my test rig. Because Cascade's two-page layout keys off regular size classes, iPad support came almost for free, and an iPad flat on the table is a good stand-in for playtesting the tabletop mode.

Cascade is coming soon to the App Store.

Gotchas to know before you start

  • Build with the iOS 27.1 SDK. An app built with an earlier Xcode runs in an inset window on Duo instead of using the full display.
  • Keep controls off the fold. Anything centered at the bottom of the screen, like a shutter or a primary button, lands on the seam when the phone is open.
  • Watch your App Store metadata. Before launch, Apple rejected metadata that named the unreleased hardware. Separately, Cascade was rejected because the word "free" appeared in a screenshot (guideline 2.3.7). Check your screenshots before you submit.
  • The simulator gets you most of the way. Layout, folds and postures are all testable. Camera behavior across the fold is the one thing I couldn't verify without the device.

Where to start with your own app

  1. Get regular width right first. NavigationSplitView, readable widths and landscape support. Most of my Duo work was this groundwork.
  2. Find the one moment where opening the phone adds something. A reveal, a browse, a table between two people. Design that moment well rather than spreading the layout everywhere.
  3. Keep everything important off the seam.
  4. Leave the closed experience alone. Most of the time, people will use it folded.

If you're adapting your own app for iPhone Duo and want a hand, I take on a few client projects at a time. You can reach me at stephen@dxn.studio.

Until next time, design for the way people actually hold the thing.