Getting My iPhone Apps Ready for iPhone Duo

Posted on: October 24, 2026

When Apple announced iPhone Duo in September, my first thought wasn’t “I want one.” It was “how many of my apps are going to look wrong on that?”

I build small iPhone apps alongside my web and app development work, all of them with Expo and React Native. They’re simple: a recipe scaler, a fuel cost calculator, an invoice maker, a couple of games. None of them were designed with a phone that opens into a 7.6-inch screen in mind. So before iPhone Duo arrived in shops on 23 October, I worked through my catalogue and updated nine of them.

This post is about that process: what changes when an iPhone folds, what I prioritised, and why I think small, focused apps are actually well suited to this new kind of phone.

What iPhone Duo changes for app developers

iPhone Duo has two screens: a 5.4-inch outer display for quick use when it’s closed, and a 7.6-inch inner display when it’s open. For developers, a few things matter:

• Your app has to move between screens cleanly. Apple’s iOS 27 reflows content as the phone folds and unfolds, and moves it to the right display when you flip the phone over.
• Both screens have the same aspect ratio. Apple designed it this way so content scales proportionally when you open or close the phone. That’s a real help. You aren’t designing for two totally different shapes, as you would with some Android foldables.
• Split View has come to iPhone. People can now run two apps side by side on the inner screen, so your app might only get half of it.
• Navigation moves to the side. On iPhone Duo, iOS puts the Dock and some navigation controls along the side to free up vertical space, so layouts that assume everything sits at the bottom need checking.

Apple has new and updated APIs in the latest SDKs for this. Xcode support for iPhone Duo in Device Hub is still listed as “upcoming”, so for now a fair bit of testing means trying layouts at the different sizes and checking them carefully.

How I approached the updates

I had a simple goal: each app should feel natural on both screens, without turning a one-job app into something complicated.

In practice, that meant checking things like:

• Do layouts stretch sensibly? A calculator that looks right on a normal iPhone can look oddly sparse on a 7.6-inch screen.
• Does anything break mid-fold? If someone is halfway through typing an invoice line or logging a set at the gym and opens the phone, nothing should be lost.
• Does it still work in half the space? With Split View, the app needs to hold up at narrower widths as well as wider ones.
• Are the buttons still easy to reach? Bigger screens move things further from your thumb.

Doing it with Expo and React Native

People sometimes assume you need native Swift code to support new Apple hardware properly. For apps like mine, React Native was a good fit for this kind of update, because the layout tools already think in terms of screens that change size:

• Flexbox layouts stretch and shrink with the space they’re given, so a layout built with flex rather than fixed widths already copes with a much bigger screen.
• useWindowDimensions updates when the window size changes, so the app can react when the phone unfolds or when someone drops it into Split View.
• Safe area insets (via react-native-safe-area-context) matter more than ever, now that iOS moves some system controls to the side of the screen on iPhone Duo.
• One codebase for everything. All nine apps share the same approach, so a fix I worked out in one app could go straight into the others. With Expo’s build service, getting the updated versions out to the App Store was quick.

The nine apps that now support iPhone Duo

Here’s what’s been updated, and where I think the bigger screen helps most.

Kitchen apps. Recipe Scaler, Cook’s Converter and Air Fryer Timer are the ones I’m most pleased with on the new hardware. A phone propped open on the worktop is far easier to read from a step away. With Split View you can scale a recipe on one side and convert the cups to grams on the other.

Fitness. My Workout Plan is a no-subscription workout log. On the outer screen it’s quick to tick off sets between reps.

Travel and driving. Fuel Calculator for UK drivers, with a journey log and CSV export, and Worldwide Tip Calculator, which suggests tips for 25+ countries and splits bills.

Work. Simple Invoice Creator makes PDF invoices with VAT for freelancers and small businesses. Being able to check every line item on a bigger screen before sending is a genuine improvement.

Games. Paws and Leap, a tap-to-jump cat runner, and What Would You Rather. The second already used a split-screen layout, so a bigger screen you can pass around the table suits it nicely.

I’ve written a fuller, user-focused round-up of all nine apps over on Mark’s Tech Blogs .

Why small, private apps suit a foldable

There’s a lot of excitement about what big apps can do with iPhone Duo, like full Slack workspaces and video calls that show shared content alongside participants. That’s great, but I think there’s a quieter benefit for simple apps too.

All of my apps follow the same rules: no ads, no accounts, and data that stays on your device. On a foldable, that simplicity helps. There’s no ad banner to squeeze into a layout, no sign-in screen to redesign for two displays, and no sync to go wrong when the app jumps between screens. The app just shows you what you need on whichever screen you’re using.

Together with a shared Expo and React Native setup, it also meant that updating nine apps for brand-new hardware was manageable for one person.

Need an app built or updated for iPhone Duo?

If you’ve got an iPhone app that needs updating for iPhone Duo, or an idea for a new one, I take on freelance web and app development work, including Expo and React Native projects. I’m happy to talk through what it would involve. Get in touch .

You can see all of my published apps on my App Store developer page.