I don't think that you should split them as they are likely to require lots of the same functions / data. Think DRY design.
You do not want to have to maintain two codebases for one project or have to patch things twice everytime there is a change in the API.
Look closely into the $route documentation and plan your implementation accordingly: you can have an isolated path for your admin without having to duplicate anything. Also, even on the same domain you might run into some weird XHR issues that you will have to code specifically for.
In short, here's my answer: don't split it. If you absolutely want to have admin.domain.com, just do a redirect from your DNS to domain.com/admin and deal with it within a single codebase. Down the line you can slightly refactor your code to match the exact behavior you want. Have a look here for some inspiration: Dynamic routing by sub domain with AngularJS