If a rider is slain and suffers more wounds than he has on his profile any excess wounds are discounted. They are not carried through onto the monster, nor onto a second rider if the monster has two riders. Similarly, any excess wounds inflicted on the monster are discounted, they are not carried over onto the rider.
Where monsters are fighting as part of a monster pack wounds are never carried over from one monster to another. In this respect ridden monsters and chariots follow the same rules for excess wounds.