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's not really that bad. A Bicycle is basically an upside down pendulum as far as the physics is concerned, adding weight at the mid point mostly affects handling, you're not just gonna tip over like with a trolley where you can shift the center of gravity off to the side so it breaks away from you
"it's really not that bad" well then explain why I tip over after three seconds on a bike then :thonk-cri:
Lack of practice, you can do it!