I think there's a big difference between 1) exploring what characters might have been like when they were younger, how they got to be the way they are today, and whether there could be any surprises in their past
I wouldn't call that a College AU, though. If all you're doing is exploring what legitimately could have happened in the character's pasts, in accordance with canon, that's a backstory fic. It's only an AU if you change the circumstances somehow - such as having the characters meet in College rather than years later like they do in canon.
Once a fic is an AU, I agree with AYRT, it really doesn't make much difference whether there's one AU axis (the characters meet in college), two AU axes (the characters meet in college in 2018), or even more.
DA
I wouldn't call that a College AU, though. If all you're doing is exploring what legitimately could have happened in the character's pasts, in accordance with canon, that's a backstory fic. It's only an AU if you change the circumstances somehow - such as having the characters meet in College rather than years later like they do in canon.
Once a fic is an AU, I agree with AYRT, it really doesn't make much difference whether there's one AU axis (the characters meet in college), two AU axes (the characters meet in college in 2018), or even more.