How to maintain an iOS and Android version of the same project?

With Unity, you can use the same code base and project (ex; tracked in git or any other version control) and then all you have to do is select a different BuildTarget when you are ready to build. You can in fact use Unity to build for multiple platforms (ios, android, windows, mac, linux, etc) from the same project. Just make sure you have all of the project settings correctly set for each desired build targets. Ex: Company Name, Project Name, App Identifier, Icons, etc.