I think it would be a great "extra"!! Indeed a +1 for this suggestion! I'd say, yes the horses are just slower if there are 2 persons on it. That the person on the back can't do anything while being on the horse is great too!! Indeed just 2-5 horses shall be able to get riden by 2 persons
Maybe the Courser, Destrier, War Horse, Arabian War Horse (2 persons would increase the maneuver too, or?) and maybe Cataphract horse or Padded War Horse??
These horses are just suggestions, but great idea!!
Greetings