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

https://shed-wiki.win/index.php/Can_Two_Kids_Ride_on_One_Bike_with_Seats%3F_A_Guide_from_the_Shop_Bench

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