Quinn, you reflected on a formidable thought. Utah to the Pac10; BYU to the B12. Both go to a BCS conference, but separate ones. They could maintain a rivalry OOC (via Clemson-So. Carolina style).
The PAC10 appears they would not be interested in BYU due to BYU's theological doctrine focus and perhaps no-Sunday play, and may not want two schools from Utah. Taking just Utah, the PAC10 would have stronger marketing options elsewhere such as in Colorado.
Taking BYU, the B12 picks up a contiguous school, and a campus with fine facilities, excellent fan support, and prior success/ credentials. And, if the PAC10 did take Colorado, there's always Colorado State for a B12 replacement, and if 14 plus ever developed throughout; less favorable, maybe, options could include Air Force, New Mexico, and Wyoming.
If circumstances were ripe, Utah to the PAC10 and BYU to the B12 could be a win-win situation.
I do think though that if the Big 12 ever needed 2 schools, BYU would top the list (as they were the original #12 candidate before Baylor got forced on them). That might change if Colorado were gone and BYU would be isolated geographically. Regardless, I do think that the Big 12 would stray from the North/West and instead go with what works for them: Texas/South. So TCU might be a more attractive option, perhaps even Houston if they needed 2 schools.