Playing Multiple Households?

I don't know anything about MCCC.
I've played games like this before though.
You probably know you have the choice to set aging to "active household only" if you want to only have them age while you're playing them. I find that's my favorite way to play rotationally- I feel like I don't miss out on their lives.

What do they get up to in the background?
Mostly, they only get up to whatever they can get up to when they're on the lot you're playing with the other sim. They might chat with other NPCs in the background, for example, so they might have new people in their relationship panel the next time you play them. They also might gain skill in areas they can practice while on lot-- for example, gaining dancing skill if they show up at a nightclub where you are playing with the other sim.
They do not earn money from their careers while unplayed. They also don't gain or lose career or school progress. With the important exception of university, but it sounds like you know that already.
If they are pregnant, the pregnancy will continue to progress while they're unplayed, so stick around for the birth if you want to be there :)
Check your Neighborhood Stories settings for "My Households" to make sure you have it set the way you want.

If you don't want the other sim to show up at all while you're playing the other one, you can use a mod that controls which sims appear in which worlds. The fact that you have them living in separate worlds is handy for this. Kuttoe's HomeRegions is the only mod I have installed and I love it.

I feel like rotational play while a sim in uni is hard. I’d finish the uni play before playing Victor again (unless it’s to pop in quickly for a wedding for Victor to get his inheritance lol)

I'll just say that when I do this, I switch households between 5 and 6 pm on the last day of the term after the grades are in but before you receive them. I know that sounds precise but then they have earned their credits and finished their work. I usually have A-A+ students, so I'm picky about making sure they are fully done with the term. I rotate during that window and whenever I come back, I get asked to sign up for another term. If you want to "see the grades", you can save at 5 pm and then exit without saving after the grades are posted.

I just play it that they are all in school at the same time while they are there but I'm only following one household during a given term. I switch between households this way whenever I want and finish the university students when I want to. I have numerous different sims at uni living in the housing and I tend to play them during the fall, early winter, early spring or on either end of summer.

If you want to make it simpler, you could wait until they are done or send them to the same university in the same household in which case, they aren't rotational.

I rotate in one week increments, and with uni involved I make sure that they've done term papers/presentations before I rotate so that they maintain good grades - obviously, they go to class/do homework/study while I control them. With the two week mod, this may be less of an issue, but I don't use it, so don't know.

As NPCs in the background, they mostly just get relationships, maybe a few skills. Make sure you've adjusted Neighborhood Stories for your played sims, as well as any mod settings (like MCCC).

I am about to switch over to a more conventional rotational style of play. I will definitely be having aging on, for the active household, only. I intend to play each of the two households for one week, at a time. So that either of my celebs can attend the Starlight Accolades Show (since I managed to remove the offending mod causing all of my grief). I will see how it goes, but I won't make the switch for at least two more Sim-weeks.