They need to be in the same savefile, not just the same town. The same town can have many savefiles. And they may differ in a lot of things. Such as the first names, appearances, and traits of Unborn Babies Keaton & Ursine, or whatever traits (as well as hair & clothing) a teen\child\toddler sim ages up into. The same premade sim can also end up have different jobs and skills in different saves, as well as having different relationships. Two sims that start up not knowing each other, could end up best friends (or even a couple) in one save, but enemies in the other. Families move in and out, and sometimes new babies are spawned from sims that were already there.