How to add 2 different queries inside an FirestoreRecyclerAdapter?
Solution 1:
There is nothing built-in to combine two queries in a FirestoreRecyclerAdapter.
The best I can think of is creating a List/array of the combined results in your app code and then using an array adapter. It's not ideal, since you won't be using FirebaseUI.
Alternatively, have a look at FirebaseUIs FirestorePagingAdapter, which combines multiples pages of (non-realtime) DocumentSnapshots in a single recycler view.