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.
I can say from personal experience that the cycling infrastructure in Japan is kinda shit, but of course it depends greatly on how far you're going and what area you're in.