Wait until your baby sits up independently, usually around 12 months, so they...

https://fair-wiki.win/index.php/Can_Twins_Ride_in_a_Bike_Trailer%3F_The_Mechanic%E2%80%99s_Guide_to_Family_Cycling

Wait until your baby sits up independently, usually around 12 months, so they can support a helmet. Front seats suit smaller riders, while rear racks handle older toddlers well