Yes of course you can have more than one passenger views, and you can have one in the cupola and one on the lower floor, in fact several cabooses in the game are set up like this. See for example the Clinchfield caboose, or my own
Passenger views for the HSC P70 and caboose or
Amfleet passenger views.
Interior detailing is another matter. It depends on whether the car's modeler has provided a separate, detailed interior (second shape) or not.
Usually, cars that don't come with a passenger view will not have an interior model. You can add a passenger view but it will show the crude model that is meant to be seen from the outside of the car. Sometimes these are very crude, sometimes surprisingly good (see my Amfleet views).
In the case of the HSC caboose, the interior model is so crude that I only provided passenger view for the outside decks.
Interior models are not child nodes, they are separate geometry files that you reference in the car's blueprint. Passenger views are simply cameras that you define in their own blueprint. The car blueprint should refer to this blueprint.