Expected

Initialize multiple Firebase projects for the Coinverse Android app to allow users to download the open sourced GitHub project as outlined in this StackOverflow answer.
Allow users to setup their own Firebase project for authentication
Run the open build variant of the app and login with Firebase AuthUI
Enable a second pre-set Firebase project for shared Firestore and Cloud Functions

Observed

Auth for one Firebase projects works, throws error below when initializing a second project

 fun initialize(context: Context) {
    if (BuildConfig.BUILD_TYPE == open.name)
        FirebaseApp.initializeApp(
                context,
                FirebaseOptions.Builder()
                        .setApplicationId(APP_ID_OPEN)
                        .setApiKey(APP_API_KEY_OPEN)
                        .setDatabaseUrl(DATABASE_URL_OPEN)
                        .setProjectId(PROJECT_ID_OPEN)
                        .setStorageBucket(STORAGE_BUCKET_OPEN)
                        .build(),
                open.name)
    Firebase.setAndroidContext(context)
    initializeRemoteConfig()
}

Auth for anonymous user works when initializing multiple Firebase projects

Error when initializing two Firebase projects and attempting Google sign in.

Sign in fail com.firebase.ui.auth.FirebaseUiException: Code: 10, message: 10:

Full Log

2019-07-29 16:34:08.130 12406-12406/app.coinverse.open E/AuthUI: A sign-in error occurred.
com.firebase.ui.auth.FirebaseUiException: Code: 10, message: 10: 
    at com.firebase.ui.auth.data.remote.GoogleSignInHandler.onActivityResult(GoogleSignInHandler.java:109)
    at com.firebase.ui.auth.ui.idp.SingleSignInActivity.onActivityResult(SingleSignInActivity.java:128)
    at android.app.Activity.dispatchActivityResult(Activity.java:7462)
    at android.app.ActivityThread.deliverResults(ActivityThread.java:4391)
    at android.app.ActivityThread.handleSendResult(ActivityThread.java:4440)
    at android.app.servertransaction.ActivityResultItem.execute(ActivityResultItem.java:49)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1816)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6718)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
2019-07-29 16:34:08.134 12406-12406/app.coinverse.open E/AuthUI: A sign-in error occurred.
com.firebase.ui.auth.FirebaseUiException: Code: 10, message: 10: 
    at com.firebase.ui.auth.data.remote.GoogleSignInHandler.onActivityResult(GoogleSignInHandler.java:109)
    at com.firebase.ui.auth.ui.idp.SingleSignInActivity.onActivityResult(SingleSignInActivity.java:128)
    at android.app.Activity.dispatchActivityResult(Activity.java:7462)
    at android.app.ActivityThread.deliverResults(ActivityThread.java:4391)
    at android.app.ActivityThread.handleSendResult(ActivityThread.java:4440)
    at android.app.servertransaction.ActivityResultItem.execute(ActivityResultItem.java:49)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1816)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6718)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
2019-07-29 16:34:08.211 12406-12406/app.coinverse.open E/AuthUI: A sign-in error occurred.
com.firebase.ui.auth.FirebaseUiException: Code: 10, message: 10: 
    at com.firebase.ui.auth.data.remote.GoogleSignInHandler.onActivityResult(GoogleSignInHandler.java:109)
    at com.firebase.ui.auth.ui.idp.SingleSignInActivity.onActivityResult(SingleSignInActivity.java:128)
    at android.app.Activity.dispatchActivityResult(Activity.java:7462)
    at android.app.ActivityThread.deliverResults(ActivityThread.java:4391)
    at android.app.ActivityThread.handleSendResult(ActivityThread.java:4440)
    at android.app.servertransaction.ActivityResultItem.execute(ActivityResultItem.java:49)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1816)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6718)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
2019-07-29 16:34:08.261 12406-12406/app.coinverse.open E/SignInDialogFragment: Sign in fail com.firebase.ui.auth.FirebaseUiException: Code: 10, message: 10:

Implementation

MainActivity

if (BuildConfig.BUILD_TYPE == open.name) {
        //Configures user's Firebase project.
        FirebaseApp.initializeApp(
                context,
                FirebaseOptions.Builder()
                        .setApplicationId(APP_ID_OPEN_PRIVATE)
                        .setApiKey(APP_API_KEY_OPEN_PRIVATE)
                        .setDatabaseUrl(DATABASE_URL_OPEN_PRIVATE)
                        .setProjectId(PROJECT_ID_OPEN_PRIVATE)
                        .build(),
                OPEN_PRIVATE)
        //Configures shared pre-set Firebase project.
        FirebaseApp.initializeApp(
                context,
                FirebaseOptions.Builder()
                        .setApplicationId(APP_ID_OPEN_SHARED)
                        .setApiKey(APP_API_KEY_OPEN_SHARED)
                        .setDatabaseUrl(DATABASE_URL_OPEN_SHARED)
                        .setProjectId(PROJECT_ID_OPEN)
                        .setStorageBucket(STORAGE_BUCKET_OPEN_SHARED)
                        .build(),
                open.name)
    }
    Firebase.setAndroidContext(context)
    initializeRemoteConfig()
}
// If build type is not 'open' configures Firebase Project from google-services.json in 'debug' src set.

SignInFragment

//Returns the relevant Firebase project.
fun firebaseApp(isOpenPrivateUser: Boolean)=
    if (BuildConfig.BUILD_TYPE != open.name) FirebaseApp.getInstance()
    else if (BuildConfig.BUILD_TYPE == open.name && isOpenPrivateUser)
            FirebaseApp.getInstance(OPEN_PRIVATE)
    else FirebaseApp.getInstance(open.name)

confirm.setOnClickListener {
    startActivityForResult(
        AuthUI.getInstance(firebaseApp(true)).createSignInIntentBuilder()
                .setAvailableProviders(listOf(AuthUI.IdpConfig.GoogleBuilder().build()))
                .build(),
            RC_SIGN_IN)
    }

override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
    super.onActivityResult(requestCode, resultCode, data)
    if (requestCode == RC_SIGN_IN)
        if (resultCode == Activity.RESULT_OK) {
            homeViewModel.setUser(getInstance().currentUser)
            dismiss()
        } else {
            Log.e(LOG_TAG, "Sign in fail ${IdpResponse.fromResultIntent(data)?.error}")
        }
}

Solutions

Issue 1

Each package may only be associated with one SHA-1 key. When removing the additional Firebase project and reverting the shared Firebase project to it's original state with the SHA-1 key the console provided the following error which revealed an issue.

An OAuth2 client already exists for this package name and SHA-1 in another project

This error message did not show originally, and would be useful to see the first time the same SHA-1 key is detected in a Firebase project with the same Android package name.

Solution

Remove the SHA-1 key from any other Firebase projects with the same package name. When using two Firebase projects in an Android application, they cannot use the same SHA-1 key.