If you added the rider and mount wound together and you attack agains the highest WS, toughness armour save and ward would be quite nice but maybe a little too overpowered for some WoC combos with absurdly high armour and ward save to give him so many additional saves. That way you would fix the one cannonball making two hits, rider and mount. About the cannons the real question is: How many points worth of cannons (hard counter to monsters) should be balanced to kill a 250 point monster in one round of shooting?
This is really the fundamental question regarding how GW point-costs their units. 1 90pt great-cannon shot can potentially take out both the Elf-prince and the dragon he's riding on. Its THAT extremity that should not happen. Either the cannon ball hit the dragon (most likely) or it hit the rider. How both? Maybe if the d6 wounds were allocated against the two elements randomly? But I think they have to deal with the cannon pin-point accuracy most of all. It is just too ridiculous, even for "fantasy". When I go to tournaments, I always have to build my lists around stopping cannon balls aimed at my heroes and monsters. It should not be that way.
right, its always rock scissor paper about "cannons or not cannons" never "what list would I actually like to bring for this tournament? A Cannon is never a bad choice, but it dictates if monsters or lone characters are. this has to balance out.