You have to spot direct the characters to the seat of the bike.
Show helpers > Spot directing > Switch tool: Move
I would use “horse” animations, so to make the characters really look like they are on the bike.
E.g. ride_horse_gallop_happy_loop at the front seat, and ride_horse_gallop_passenger_happy_loop for the character at the back to look like he/she is holding onto the character in front.
You can zoom up to the upper half of their body to make it even more realistic.