A second 3D model is necessary usually when one level is not internally connected to the other levels. Example: the basement can only be accessed through an exterior door (exterior side door etc.). This usually happens when a house has no interior access to the basement, or the interior access is locked.
It is technically impossible to stitch 2/ or more levels together without interior access to each other. That’s why the second model is necessary. When a second model is added, the separated level can be added as an individual unit to the virtual tour. (Sample 2-model home: https://www.houssmax.ca/vtour/c4371186)