Wait until your baby sits unassisted with steady neck control, usually around...

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 unassisted with steady neck control, usually around age one. Front or rear seats suit toddlers best. Trailers accommodate infants using specialized inserts