Even in your scenario, you outline it as

1. Tell PAC you're only taking one team
2. Get one to jump for that reason
3. Use that as leverage to get a second team

If another team is of value, the Big 12 would want to add them. If another team isn't of value, the Big 12 wouldn't want to add them.  

Nobody believes the Big 12 will pass on schools that would make them better, just as punishment for their schools not jumping earlier. And frankly, the Big 12 shouldn't do that