This is actually a very difficult thing to implement in most games, much less one that was coded so strictly as Dragon's Prophet.
As Aiden said, there would need to be another mount point, and then you would need to figure out how to mount on each dragon, and also program the logic for when there are two people seated, or only one. You would then need to make it so that a game recognizes movement that is under the control of the driver, and the only vehicles in DP are trains/airships and they have their own complicated system that is unlikely to be reusable.