my original suggestion was primarily meant as a way to transport people, not as a platform for fighting.
It doesnt break realism either, riding double has been done at all times.
Like a vanguard, preparing for the the mainforce, scouting. Less mindless running.
would make the fighting more dynamic, expands cav role , makes horses more valuable, offers plenty of new teamwork variants.
Rescue ppl from tough spots, reinforce weakspots, move key players to where they are needed.
And probably some interesting 2player tactics.
??Maybe 1 ridingskill required to take the "backseat", and a temporary 20% slowdown of the horses maxspeed.??
Passenger mounts like today, and demounts by jumping off, rearing causes passenger to fall off the horse.
Jumping off while the horse is running ,causes knockdown.
Ofc depending on if it is possible and wanted
I think it would be great