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.
It seems like it'd be really hard to wrangle the second kid into the seat while you're balancing the bike for the first. Big box seems easier for that.
Look at the center stand mounted on the rear dropouts - when you're loading kids the bike would sit upright on the stand.