No that is not possible. It will screw up the database because each app creates GUID etc. You could give each app its own database and a third one which holds the “true” state of an object. Then use REST services to do the retrieve and update. Not an easy feat because what if both systems try to change an object on the same time?
Is not recommended and not possible for mendix cloud databases (because you can not reach externally the database).
Techncally it is possible, still not recommend, if you can reach your database (on premise set up for example) you can use the database connector module (appstore) to execute crud operations, but be aware that Mendix has its own identifiers and mechanisms for the entities, types, etc. in the database, if you screw up some of those entities it wouldnt be usable any more for mendix
I would also (re)think integration solutions, depending on your use case, to share the data with an api or so.