Everytime divisional splits are brought up, you run into the problem of having lopsided divisions. In fact most present 12-teamers suffer from this problem to one degree or another. Putting 2 teams in a major rivalry in different divisions could balance out the conferences.
For example, I'll use the Pac-10. It seems to have the perfect pair-up of archrivals. Say you put USC, Cal, Washington, Oregon, Arizona, and an expansion team in division A; and division B has UCLA, Stanford, Washington St., Oregon St., Arizona St., and the expansion team's archrival.
Since rivals are generally evenly matched and close to equal, by putting two halves of a rivalry in different divisions, you balance out the interdivisional power a little more. You have less disparity issues like Big XII North/South.
Scheduling wouldn't be that big of a problem since there are usually 3-5 interdiv games, and 5 divisional games. All the rivalries and important games with teams in another division can be scheduled fairly easily. SEC has many yearly interdiv rivalries like Georgia-Auburn, and Alabama-Tennessee, and these haven't suffered that much from a divisional split. Sometimes a rivalry rematch occurs, which I don't see anything bad about.