title: Change default firebase database programmatically and run the same firebase functions both on default and secondary database
body: <p>Background - 
  I'm setting up a new feature that allows users to select which city they are in, since my app is a public transportation app. I would like cities to be in separate databases, and for that I have created a secondary database in my firebase project. It's a React Native App and I'm using react-native-firebase.</p>
  
  <p>Question 1 -
  When the users selects a different city, I would like that database to be his default database. I couldn't figure out how to do this, can anyone help? </p>
  
  <p>I've tried initializing and just changing the databaseURL, and even though I connected to the second DB once, it didn't do so every time. Seemed like an unstable solution. </p>
  
  <p>The other solution I found would be to pass the url to every "firebase.database(url)", but that seems like a bad solution. </p>
  
  <p>Question 2 - 
  Since the app is exactly the same on both cities, I would like to run the same functions I already run on DB1, on DB2. They are completely separate DBs, but with the exact same nodes. Both have a "location" node, for example, and then I have a listener for changes there. How do I set up listeners on both databases with just one function? Or do I need to get a reference to DB2 and just duplicate the functions?</p>