In the early 90's, when the B12 was forming (from the Big 8 and the remnants of the Southwest Conference), BYU was seriously considered to be invited. If BYU had been invited and had accepted, then the UofU would have understood, just like BYU understood when the UofU accepted the PAC12 invitation.
What I don't think would have happened, however, was that BYU would've gone independent at any time without the UofU leaving first. BYU had thought of independence several times over the years but never would pull the trigger as long as the UofU was still in the same conference.