I say unusual in the sense of Japan seems to be the only one to have solved the issue of how do you transport 2 kids on a bike not with "big box" like the dutch and subsequently the entire western world but via 2 seats, they're apparently very popular over there as a means of transportation.
Good point. Still seems dangerous for the kid in front to me because they don't have anything protecting them except for a helmet, but I guess no more than another collision on a bike.