Maybe there’s a cultural idea about mirrors being somehow “the same”. After all, a mirror shows the same thing regardless of which one it is. Or related in cultural mythology to a singular adjoining world that contains your doppelganger (in such media, you don’t usually have a separate mirror-self for every mirror, but one that can be accessed from any mirror). Also could be a turn of phrase that stuck without a good reason.
That seems weird, the opposite position makes more sense to me. You can’t think of any possible economy where you could morally have two houses, and in this situation it’s somehow necessary? Could you elaborate further, because it seems reasonably plausible that there could be an economy with significantly more houses than households, to the point of warranting multiple ownership. And of all the things to call second house ownership (convenient, luxurious, smart, excessive, warranted), necessary isn’t the one that comes to mind.