The way I'm picturing it in my head, one kid has most of the shoulder, all of the lap belt. The other kid gets the bottom part of the shoulder, and has the lap belt. The kid with the shoulder belt would be ok and restrained, but the kid that got the short end of the shoulder belt would be thrown forward (and hopefully not out of the seat) in a serious rear-end collision.
Look, I live in California, and I have a rule that your seat belt better be on before my car starts moving. I also refuse to transport more people than I have seat belts for in the car. I KNOW what the laws are in this state regarding seat belts and that it is a primary offense, and I don't want to be screwed over with the fines, penalties, and corresponding increase in insurance. I do NOT enjoy being pulled over by the cops and I drive very tamely to avoid these occurrences.
The actual fine is probably less than half of the total cost per ticket. It's all the fees that the courts, state, and county imposes on top of a ticket that makes it crazy. Blame that on the broke-assedness of CA.
This is a state that uses the Amber Alert signs to flash "Click it or Ticket" warnings. Everyone here in CA should know better. If you don't, just be prepared to lawyer up or pay.