If you haven't read this already, have a look at it. In short it looks like you can only achieve this using extensions at the moment.
This article explains what they expect from you as a 3rd party watch app developer.
Apple Watch requires the presence of an iPhone to run third-party apps. To create a third-party app, you need two separate bundles: a WatchKit app (that runs on Apple Watch) and a WatchKit extension (that runs on the user’s iPhone). The WatchKit app contains only the storyboards and resource files associated with your app’s user interface. The WatchKit extension contains the code for managing the WatchKit app’s user interface and for responding to user interactions.
Also
Because a WatchKit app extends the behavior of your existing iOS app, the WatchKit app and WatchKit extension are bundled together and packaged inside your iOS app bundle. During installation of your iOS app, the system prompts the user to install the WatchKit app when a paired Apple Watch is present.