UNC-Charlotte, UNC-Chapel Hill - Only one is really considered "UNC"
UC-Davis, UC-Berkeley - Only one is considered "Cal"
UT-El Paso, UT-Austin - Only one is considered "Texas"
and a kabillion others. One is known as THE school usually the first, biggest or richest, and the others are considered fine alternatives. I like that BYU has a university system. It's ok to say "Cal Berkeley" or more commonly "Berkeley"
If someone says "UT Austin" the Longhorns are okay with that too.

SO, in my opinion, its okay to say "BYU-Provo".

But the other way around is not too common. If I am going to UT-Dallas I don't get to say that I'm going to the "University of Texas". I have to add the Dallas.

SO, also in my opinion, its not okay to say "BYU" when you are talking about "BYU-Idaho"

sorry. way too long.