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.
This seems to be the only time the overengineered Canadian/American solution is the best one: don't stick your kid in a wheelbarrow on the front of your bike or in a carrier seat in front of your handlebars. Put them in a trailer with an integrated roll cage.
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Honestly they're not that bad. My dad had one for my sibling and I when we were little, and I've used them with my ebike when babysitting family members. If you have big ol' ebike disk brakes it's no biggie. Dad said it was only a pain to control for tight turns or stopping downhill. The worst part is just added weight going uphill mostly.
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