Deon,
In Appery - once an app is tied to a database and it's notification related data (**the devices table) in Appery - it can't be tied to another App.
Appery manages the user devices that are connected to the App in the same DB that users are managed from for the App - thus the restriction. (** the tokens are generated by an App specific push notification call - and it's difficult to manage in a canned way - this relationship across multiple apps and multiple databases - so Appery does not).
If you want to use the same DB for notifications that another app is using you can:
Manage the devices in the 'other' database 'manually' using functions in the push notification plugin - and get device UUID's and tokens for the devices and add/update/delete them to the devices table in the DB manually - however - that's likely a layer of complexity you don't want to deal with - but it can be done (we've done it where it was a requirement to do so).
You can send all of your notifications from server side scripts using the shared key.
Using these two approaches - you can 'defeat' the limitations - but not a task you would take on lightly or perhaps without some additional assistance.
Best,
Bruce