Two people riding is a good idea (picking up a buddy), the horses should move like 15-25% slower (IMO, even if it's not realistic for balance reasons). The person in front who is the normal "driver" should still be able to attack forward, but not to the sides. The person in the back should certainly be able to attack.
But honestly I'd be okay with just making it so the person getting a ride can't attack, and they are just a passenger. Only problem with this suggestion, you have to stop the horse completely to get off of it. Not sure if I've made a thread for it or not, but I'm going to suggest being able to dismount while a horse is riding (you can mount them while a horse is moving)