They run the same underlying OS, in many ways it doesn't matter which replaces the other, the UI chrome and API are the main differences, and something like xco

They run the same underlying OS, in many ways it doesn't matter which replaces the other, the UI chrome and API are the main differences, and something like xcode could be run on iOS without huge modifications. I do think at some point they'll merge them though - maintaining two APIs which are distinct but incredibly similar is a waste of effort and confusing for developers.