The answer to the underlying question though is that without mod usage only one household can live on each residential lot no matter how the buildings upon it are arranged. There are two types of apartments in TS3. The LN kind with one usable residential space and darkened out NPC units for what would be considered "homeless" NPCs to hibernate in, and the Uni kind that forces the town's Roommate Service to send over up to 7 NPC Roommates that share the resident sims' living space. Neither one allows for multiple resident households on the same lot.
Sims cannot live on a Commercial lot without a mod to allow for that, and again it would be one household per lot.