Do you mean iOS or visionOS? It’s definitely possible to use the same basic codebase (for SwiftUI, e.g.) in iOS and visionOS, and use compiler directives for platform-specific code. Similarly, you can build Unity projects that can build to both iOS and visionOS.
The best way to do this at the moment (AFAIK) would be to use a Unity app that includes SwiftUI elements (as in the link I provided). In that case, I believe you can have the Swift developer iterate on an Xcode project generated by Unity, and copy the Swift files back into the Unity project. In that setup, Unity is responsible for the “main” part of the application.
The other option, where the “main” part of the application would be a standard Swift/Xcode app, would be Unity as a Library. That’s not something we officially support for visionOS, but I believe that some developers have managed to get it working. If that’s a feature you’re specifically interested in for visionOS, I’d suggest submitting it as a priority to our road map.