Hi Sonnich
I think this will need a bit of a workaround.
I would create an external datsource "Notfications" with a colum something like "VisibleToUser"
Whenever a User in Building A does the action you want, you create a new record in your Notifications datasource.
There you can populate the VisibleToUser Column with the user from Building B.
In your app you will need to regulary refresh the Notifications Datasource. You can use a timer and Refresh().
Then you will need a component to display the notification in your app.
I suggest component so it is easier to maintain on all screens.
You can create the component with logic so it will only be visible if there are Notifications in the Notifications Datasource and if the logged in User is the one in the VisibleToUser Column.
You can delete the record from the datasource whenever the user has dealt with the notification.
If you need further help with example code, let me know.