In application development beyond Domino, the preference has been for looser coupling. In Notes Client there is a lot of tight coupling. Composite Applications aimed to address this, but they were complex, tied to Eclipse, and crucially required hard-codings between NSFs that were tied to implementation and environment.
This means that although applications can be split into multiple databases:
Having the UI and business logic in a separate database would also make sense in the context of LotusScript REST services, to have all your REST services in a single NSF and re-using client business logic, while restricting web access to all the data NSFs.