thats not posible because the once was been player model will be turned into a vehicle model then.
Why would it not be possible? The horse is an AI/player and the saddle is the vehicle. The horse mounts the saddle "vehicle" by being under it, the gunner mounts the vehicle by sitting on it.
The gunner animations seem like they would be somewhat difficult and there is the question of would the "horse player" walk when moving in the "saddle vehicle".
That might a problem. But there's probably somebody out there smart enough to figure it out.
How about this? Is is possible to have a horse that "gets in" a saddle vehicle. And when that happens, the player/AI horse is hidden and replaced with a vehicle horse with saddle? Of course, if a human AI/player got in, it would look like they magically became a horse, so maybe that's a bad idea. But there could be something useful in there somewhere.