I think if they did this split they would not put Oklahoma and Texas in the same division, they'd be the crossover rival though. Even with Texas down, they're a money maker and the conference is going to want OK/Texas in the football championship game every year.
It was rumored a while back that OU and UT did not want to be in opposite divisions because they didn't want to play twice in one year. I think that was from Chip Brown or Orangebloods but I could be wrong.
My point is more that I see two Texas schools in each division if the Big 12 expanded to 12.