When a shooting attack hits a monster and rider, its randomly distributed, not distributed back and forth (this is from the BRB under monstrous mounts in the character section).
Roll a D6 per wound dealt, on a 1-4 it wounds the mount, on a 5-6 it wounds the rider. Characters on chariots are distributed the same way.
PG 105 "Shooting at Ridden Monsters".