Can iPhone Duo run iPad apps?

No. iPhone Duo is an iPhone. It runs iPhone apps from the App Store, and iPad-only apps are not installable on it any more than they are on any other iPhone.

That is the literal answer, and it is also the less interesting one.

The question underneath

Most people asking this have noticed that the inner display is 7.6 inches — tablet territory — and are really asking “will my app get the iPad treatment?”

Here the answer is more useful: the inner display reports Regular width and Regular height. That is the same size-class environment your iPad layout already targets. Apple’s own guidance notes it leaves room for sidebars.

So while iPad apps do not run, your iPad layout very often should:

@Environment(\.horizontalSizeClass) private var horizontalSizeClass

var body: some View {
    if horizontalSizeClass == .regular {
        SplitLayout()      // the branch you built for iPad
    } else {
        StackLayout()
    }
}

The trap that stops it working

If your wide layout is gated on device idiom rather than size class, it will never appear — iPhone Duo reports .phone:

// Never true on iPhone Duo, however large the screen
if UIDevice.current.userInterfaceIdiom == .pad { … }

Teams with an existing iPad target are often surprised how little work remains once that gate is removed. See idiom assumptions and size classes.

What about Mac Catalyst or visionOS builds?

Neither is relevant here. iPhone Duo runs iOS 27, and the artifact it installs is an ordinary iPhone build. Nothing about the foldable changes which platform binary is used — only how much space that binary is given.