Have two divisions like you say. Probably fine just to have 12 teams total. Work out a deal with the MWC and the AAC for relegation, MWC for the West division and the AAC for the East division.

You'd have to get an NCAA rule change to actually allow the worst team in each division to play the best team from the MWC or the AAC at the end of each year. But if you can't get the rule change at first, just have your top lower-division teams play in a bowl for the right to enter the Big 12. It would be a cash cow. ESPN would love to televise it.

Better yet, start off that way. Simply announce that the B12 has made the arrangements, but the two new members are undecided yet. BSU-BYU and Cincy-UCF will be playing each other in their bowls for the right to join. The revenue would be huge, on par with a CCG.