I would hope that the committee wouldn't put 2 teams from the same conference in the playoffs. In the case of the B1G, ESPN wouldn't breathlessly lobby for that the way they lobbied for multiple SEC teams for the playoff last year.
It's going to have to happen. In most years, at least one P5 champion has 2 losses. It's rare for all 5 to have 1/0 loss. That essentially means one conference will be out right off the bat. Then you have no guarantee that the other four will have a playoff contender among them. (For example, in the last 5 years, there have been multiple 2-loss P5 champs 3 times.) That means that there will be several times in which one spot is open to an at large team, most likely from the same conference.