The Xamarin stacks include both Xcode & Android SDK, but the updates are tied to Xamarin updates
You can definitely add the same repo multiple times, e.g. once for iOS and once for Android - that way you can have better control over e.g. the Xcode version you want to use, or utilize features which are only available on the Linux/Android stack like downloading the whole environment as a docker image (which is not possible to do with macOS, only with a Linux image) Running your build locally in Docker - Bitrise Docs