If you want to be able to control two (or more) sims at uni, they both need to be within the one 'household'. At least that is without mods/cheats.
Having said that, you can choose to have them not related to each other by blood and/or claim they are strangers who happened to move into the same dorm.
You can also befriend existing sims in university while there, and I guess invite them back to your homeworld to interact further with them, and have them become part of your household.
Mods may exist that achieve closer to what you want, but I don't use mods in my game, and I really don't know, sorry.
Sorry I couldn't be of more help,
@Peanuts61