1. Hardcoded
2. Possible. Just make the unit a chariot type with only one horse and 2 riders. Edit the chariot model to make ot practically invisible. Positioned the 2 riders to sit on the horses by their xyz coordinates. Now you have one guy ALWAYS holding the reins and another guy throwing/shooting/attacking. Of course, you'll need to modify the animations to show them properly "sitting" on the horse. For the riders, just swap the entries with the entries of a regular horse rider's anims. No need to create custom anims unless you're really picky and want the first rider to be more animated.

Note, however, that if the unit has a general/captain present, he'd take over the second rider's spot.