There is not a way for Mendix apps to share databases as this would lead to database integrity issues. However, you can designate one of your apps as the system of record for customer information and then build Rest APIs to enable your other apps to interact with the system of record (i.e. create, read, update and delete customer info as needed).
Hope that helps,
Not if your applications are hosted in Mendix cloud, because Mendix does not allow. But may be you can try the same if applications are hosted in external cloud like AWS. I have been in a discussion with my cloud team recently about the same topic, where in-theory seems possible. Yet to try this ourselves.
But you must take care multiple things when you are doing this like: database tuple locking, transaction deadlocks etc which might be hard to manage this across multiple applications. If it is like 1 app writes and other apps reads, then should not be a big problem.
What Mike suggested is a good alternative if you want to read-write on same database from multiple apps.