Riders per mount: hard-coded?

I've been trying to understand the parameters present in descr_mount.txt, with the objective of creating a unit with two riders on the same horse.

The first attempt was simply copying and renaming medium_horse, and giving it 2 riders, as it happens in the small elephant. The offset was changed but the game gave an error, saying that it was expected a horse (I imagine it was due to the number of riders).

Thus I tried to understand how it works for elephants. In fact the small elephant has one only rider (it is a truly rider), but the model in descr_mount says 2 riders are possible. Thus I duplicate the carthaginian_elephant_forest unit, giving it 12 riders, for 6 mounts. This is the result:
http://img68.echo.cx/img68/6126/gracia33km.th.jpg
Thus I wondered if all the riders, apart from the first one, would be stand, as it happens in chariots and bigger elephants, but I remarked this:
http://img68.echo.cx/img68/949/gracia46tg.th.jpg
The general is also riding the elephant!

Does anybody know where it is coded if the rider is riding or standing?

Finally I tried to cheat the game, by saying it that the new horse was in fact an elephant. When class is elephant, the game needs a lot of information. I copied all that information from the small elephant. The game starts and I can select the unit. However the soldiers aren't visible (only the horses) and the game inmediately crashes when the unit begins to move.

Any idea?