Shared database is, in fact,a classic enterprise integration pattern, and much of classic relational database engineering assumes that multiple application will share the same database; in the classic ideal, each would use it through a set of (read/write, ad needed) application-specific views so as to avoid exposing implementation details of the base tables and to permit each application, and the base database, to change with minimum disruption to any of the others.

Reading data from another application's database is pretty common (although even this can cause chaos if done without some care) but writing to application databases is often a very bad idea and often explicitly forbidden by CRM/ERP vendors.