Sims 4: How can I make multiple families live in an apartment complex?

Yes, just create a new household on the world screen and move them in.

You can lock doors for people, so if you make sure to lock the door for the right people they'll always enter the proper apartament, but you'll have to unlock the doors for them if you want them to visit each other. And of course you'll have to restrict yourself to 8 sims in total (unless you install a mod to increase max nr sims per household), because you can't have two households on the same lot. (but families yes)