I'm not Wooly, but I'd ague that most places with multiple studs on the two-deep aren't in a such a strange situation in terms of "Age vs Experience."
Usually, there's a pretty clear pecking order. The older guy starts if he's a stud; the young guy develops and waits his turn, or else he unseats the older guy.
ND's situation, on the other hand, has been completely unstable since BK came in, due to circumstances outside of anyone's control. ND hasn't had a chance to establish that hierarchy. Thus, two-QB system is the unique answer to having two studs in this case. Kizer is younger, but has more experience; Zaire has less experience, but he's older and won the job a year ago. THAT is why more teams don't do it... they don't have the unsettled situation. And if they do, they don't have the talent that ND has.