Wait until your child sits up and supports their own head, usually near age...

https://city-wiki.win/index.php/Can_Two_Kids_Ride_on_One_Bike_with_Seats%3F_Everything_You_Need_to_Know

Wait until your child sits up and supports their own head, usually near age one. Front seats work well for toddlers, while rear seats suit older kids. Trailers offer a secure, reclined ride for the youngest. Check that helmet fit and enjoy the trip.