Yeah, if the two buildings are physically connected by a corridor then they should be integrated. Looking at my imagery source again (this one isn't available in iD/JOSM), I think what happened is that I mistook the corridor for a concrete footway.

If we wanted to still represent parts with different heights / roof types / etc, the correct way is to put building:part regions inside of the outline afterwards. Doing that kind of thing is not a very high priority though, it's pretty much only used for things like 3D views in apps.