Apartments

AngelBaby - apartment and multi-family residences can't exist in The Sims 2 in any sort or realistic sense.

Depending on what expansion packs you have installed - some custom objects may help, but if you have Open For Business and later EPs, these custom objects cannot be added to the game without major conflicts.

The best way to achieve this is through the "illusion" of separate dwellings on the same lot. Others can probably explain this better than me, but it is largely a matter of training the sims to sleep in one particular bed and making sure the two dwellings are far enough apart that a sim won't walk to the next apartment or house on the lot to wash dishes or eat. If the 3x5 lot is avalable in your game, this is probably the best lot to use for this -- place the dwellings at the far ends of the lot from one another.

In the university EP (and only in university neighbourhoods) you can create dormitories, but the "illusion" is paper thin and leaves much to be desired.