I'm not very optimistic about two Big12 teams in the playoff, even if the championship game is between two unbeaten teams. My reasoning:

There are only 12 spots. 5 go to conference champs (B10, SEC, ACC, Big12, Group of 5). So, let's use the scenario of BYU wins the Big 12, they are undefeated and get a bye as a top 4 seed. Woo hoo.

Now, let's assume Iowa State lost to BYU in the conference championship game. They are, then, 12-1. They are fighting for one of the remaining 7 at large spots. Do you think they would get in over:

The loser of the conference championship game in the B10 or SEC? Regardless of their records? No.

So, of the remaining 5 spots. Would a 1 loss Iowa State team get in over:

ACC Runner up? If it's 1 loss Miami? No. 2 loss Clemson? It could be close, but the committee may love Clemson...
1 loss Notre Dame? No.
1 loss Penn State? No
2 loss Georgia? No
2 loss LSU? No
2 loss Alabama? No
2 loss A&M? No

What I'm saying is it will be tough. That's why BYU's best chances are the auto bid, and the best way to get that is to hope for a clear path to the conference championship game and pull out a win there.