Combine Two Android App Source Codes into one

I need a skilled developer to combine two Android mobile application source codes into a single application.

Additional Features:
- Add basic functionalities such as user authentication, push notifications, or in-app purchases.

Ideal Skills and Experience:
- Proficiency in Android app development
- Experience with source code integration
- Familiarity with implementing user authentication, push notifications, and in-app purchases

Please provide samples of relevant work and a brief outline of your approach.