It was my impression that such an arrangement was done with an intervening floor between the levels. For example there is One King West in Toronto with a much-photographed round-door vault, which is on top of a simpler door vault one floor below. The arrangement economizes construction costs by having a common foundation (and possibly a thin floor inside the vaults separating them). Later, that intervening floor might be removed to “open up” the space when the building is re-used for other purposes such as a Target store.
Were there actually such “double stack” doors in normal use?
Another example is a hotel in Milwaukee where the upper doors are less impressive and, again, I assume a floor was removed to open up the lobby:
