Theoretically, if MBS does leave the locking to be handled by the database engine (Postgres in my test) having two apps connect to one and the same database should be possible, so I tried this. I just did and get further than expected: two apps running locally using the same database. It even works ok. I created one entity and overview pages and I am doing CRUD actions without any problems so far.
That would have great advantages, but the danger of database corruption is around the corner. Is it currently possible for Mendix applications: apparently yes. But you have to be able to connect to a specific database of your choise. Not all cloud solutions allow you to do so. Biggest hickup might be to get an ok from Mendix. I don’t think they will allow it, but it is worth a shot to ask them.
Mendix has created the DataHub which does allow apps to connect to the DataHub, serving as the database, but it is only for reading, so far.
In case worries about handling high traffic are the reason for your question: you can consider horizontal scaling. see https://docs.mendix.com/developerportal/deploy/scale-environment