Mobs with passengers don't collide with others

Mobs that have any(as far as I know) entity as a passenger do not follow the collision rule with other mobs with passengers. Whether the passenger has AI or not does not seem to affect the situation.
Example: If two zombies with passengers path-find to the same spot then they will stand in the same spot. These zombies will appear to be one. even with hitboxes shown, their hitboxes line up exactly.