iPhone Duo apps: what changes for your users, and the seven updates an existing app needs

Closed, it is a normal phone. Held upright, that is the screen every iPhone app was designed for. Turned sideways, it is new: a small, wide screen no iPhone has had.

Open, it is the size of a small iPad, either way round. An iPhone app on that screen does one of two things. It stretches — same layout, bigger, empty space on the right. Or it rearranges — two things next to each other, the way Mail on an iPad shows the inbox on the left and the open message on the right. Stretched is what “not ready” looks like. Rearranged is what “ready” looks like.

Two apps side by side. Apple’s newsroom: “Split View allows users to open two apps side by side on iPhone for the first time.” People will drag a photo or a link from another app into yours.

Half-open on a table, it stands by itself. StandBy — the clock-and-widgets mode — works “on either the outer or inner display — even when it’s not charging.” Apple Pencil support is announced for later this year.

For a product manager, each of those is a place a feature did not have before: a second screen, two panes instead of one, a phone that stands up. The seven changes below are how an existing app meets them.

Apple’s “Prepare your app for iPhone Duo” talk names three size-class combinations. Outer display, portrait: regular height, compact width — the layout every iPhone app knows. Outer display, landscape: compact and compact — new. Inner display: regular and regular in both orientations — the iPad layout.

The sentence that tells a product team how much work it has: if the app already works properly on iPad — two panes, rotating, side by side with another app — it is most of the way there. If the iPad version is the iPhone layout stretched, that is what everyone will see when they unfold the phone.

Each one: what the user sees, how much work, and the sentence to send your developer or type into an AI agent. The question that decides the effort on almost every line is whether the app uses Apple’s standard parts for navigation (lists, tab bars, split views) or parts built from scratch to look special — ask it first.

1. Rebuild with the iOS 27.1 SDK — effort: a rebuild, no code changes. Apple’s talk shows an older-SDK app running on the open screen at “a familiar size and aspect ratio” — a phone-shaped box — and a 27.1 app that “extends to the edge of the screen” with its bars in a vertical column. Xcode 27.1 beta has been available since September 18. Sentence: none; select Xcode 27.1 and build. This is also the change the April 2027 rule makes mandatory for every update.

2. Two panes instead of one — effort: one sentence if the app uses Apple’s parts, a project if it doesn’t. On the open screen a list should become a list on the left and the selected item on the right. Apple’s NavigationSplitView and TabView adapt to the fold, the cameras and the side column on their own; custom navigation does not. Sentence: “On the open iPhone Duo screen, show [the list] as two panes: the list on the left, the selected item on the right; keep the current navigation on the closed screen; move the tabs into the left pane on the open screen.” If the navigation is custom, this is the moment to replace it with Apple’s rather than adapt it.

3. The buttons move to the side — effort: an afternoon. The row of buttons at the top becomes a column on the side, shared with the status bar. Order matters: back or close at the top, the main action under it, the rest below; every button gets an icon and a name so the phone can choose which to show; counts become badges; extras fold into a “more” menu you control. Sentence: “Arrange the toolbar for iPhone Duo’s vertical bar: back at the top, [main action] pinned under it, the rest below; give every button an icon and a name; show [count] as a badge; keep [button] out of the overflow menu.”

4. Nothing under the fold — effort: free with Apple’s parts, about a day per custom screen. Half-folded, there is a crease across the screen and two camera holes; Apple calls them reserved regions. Standard containers move content off them by themselves. A custom screen — a player, a board, a chart — needs to be told: split it across the fold, or float one part over the other. Sentence: “When iPhone Duo is half-folded, keep [part A] above the fold and [part B] below it.”

5. The hinge — effort: an idea, then an afternoon. The app can read how far the phone is open — an angle plus whether it is closed, half-open or flat — with a few lines of code (iOS 27.1 and later). That is a new input, like the tilt sensor Maps uses. Finding the feature is a product job: a memory app that becomes a slideshow when it stands on a table; a recipe app that goes hands-free; a timer that gets big when half-folded. Sentence: “When the hinge is half-open and the phone is resting, switch to [the standing-mode feature].”

6. Shapes and sheets — effort: an hour. The front screen has uneven corners, so rounded backgrounds the app draws itself need Apple’s shape that follows them; pop-up sheets rearrange on their own but should be checked once open and closed. Sentence: “Use the shape that follows iPhone Duo’s screen corners for the card backgrounds.”

7. StandBy, drag and drop, Pencil — effort: an hour. Widgets that skipped StandBy should get it, because StandBy now works on either screen without charging. Two apps side by side means people will drag things into yours — decide whether that should work. Apple Pencil support is coming later this year; if the app has anything you draw or sign, put it on the roadmap.

Every change above has an Android counterpart, and the vocabulary is close enough that a product spec can name both in one line. Android’s foldable documentation describes the states as FLAT and HALF_OPENED and, when half-opened, “tabletop posture (horizontal fold) and book posture (vertical fold)”; the Duo’s half-open-on-a-table is Android’s tabletop posture. Two panes on the open screen is the same window-size-class decision on both platforms — regular width means two panes — and Android’s adaptive-layout guidance says so in one sentence: layouts “adapt” when the device is “folded or unfolded, in portrait or landscape orientation, or in tabletop or book posture.” Nothing under the fold has the same name on Android — a folding feature that occludes the display — and the same rule.

For a product manager this means the spec line is written once: “Adaptive layout: two panes in regular width; content avoids the fold; a standing-mode feature when half-open on a surface.” In a tool that builds both platforms from one project — in Modaal, the logic of each feature is written once and the iPhone app renders it in SwiftUI while the Android app renders it in Jetpack Compose — that line produces the Duo version and the Galaxy Z Fold version from the same PRD. The iPhone-only builders can write the first; the second is a second project. How to build a cross-platform app with AI has the mechanism; the PRD prompt has the document.

October 23, 2026 is when people have the phone (pre-orders October 16). Apple reviews every app before it goes in the store, and review takes days, so to be live on launch morning, submit by around October 16 with a build from Xcode 27.1. The store fees are the usual: $99 per year for the Apple Developer Program, $25 once for Google Play if the Android version ships too.

April 2027 is when the rebuild stops being optional: from then, every app or update uploaded to App Store Connect must be built with the iOS 27 SDK or later. An app that is not rebuilt cannot be updated after that date, on any phone.

The video version of this page, with the seven changes done live on one of our apps, is on YouTube.

Ask your developer the one question: did we build our own navigation, or use Apple’s parts? If Apple’s, the whole list above is days; if custom, change 2 is the project and the rest is days. Then send the sentences. If you build with an AI agent yourself — Modaal builds with the Xcode you select, so the 27.1 beta is a menu choice, and runs the app in the Duo simulator open, closed and half-folded — the sentences above are the prompts; the agent writes a plan you approve before it changes anything. The Free plan covers one project with unlimited prompts on one platform; Pro is €9 per user per month billed annually and adds both platforms and TestFlight distribution — /pricing. Modaal for product managers is the page for how a PRD becomes the build.