I am using Firebase with React Native. There are two different projects with one Android app under each project.
My backend server is configured to receive/store the FCM token of every individual app user. Right now, the same backend server caters to both apps. The FCM json config file for the backend server is based on one of the aforementioned two projects.
Can the backend send notifications to an user of either of the two apps, as long as it has their FCM token?
You can do this so long as it comes from the respective apps FCM credentials, this prevents abuse and FCM hijacking. this does not mean that you need cloud functions on multiple projects as you can manually instantiate new admin-sdk modules to connect and send FCM messages as needed.
A side note to consider: You may also be interested in registering the FCM if your app has multiple projects as only the default app is registered with normal behaviour.