I'm not sure that push notifications are the best way to go with that -- instead you can use the Firebase real-time database, or the Firebase Cloud FireStore, and have both apps reading/writing from that database, and subscribing to get event notifications in real-time.
However, if you really need the apps to send push notifications to each other, then I think you may be able to do that using the Firebase "Cloud Functions" service, and have one device trigger a cloud function that sends the push notification to another device.
