Thanks for the answer to my question, Kato.
I think I prefer the first option (point to a subtree on the same hosted app) since I wouldn't have to set up two separate accounts. But how do I set up the hosting part of this "subtree" app with Firebase? I take it when your use of the word "subtree" in this context you are referring to a node on the data tree. Yes?
In any case, I am still not sure how to deploy two separate apps on my Firebase account. This kind of thing is straightforward with other technologies I have worked with, even with Meteor.js and Modulus.io, both of which I am abandoning for Firebase/Angular/Polymer.