Ancalagon
Dusty Dragon
Had a big discussion on RPG stack Exchange some years ago on this BS.
It is imho to ridicolous to be even discussed. Dual wielding in RL has only been widely done with rapier / dagger nothing else (Sabre / Pistol in later times sometimes also). Everything else was just showing of skills mostly. Several myths are still around:
Most RPGs allowing dual wield asume you are getting more attacks by dual wielding making you in fact faster, that is of course rubbish since you gotta Change stances (Forward leg) to bring the second weapon to the attacker which in fact is slower than fighting single weapon or weapon shield.
Why was it done then? Well, for parrying purposes in rapier fighting, though some prefered a buckler / cape / coat / hat / good glove for this.
Normally whe nfighting with two wepons RL you attack with one and defend with the other. Everything else is just making things complicated.
Yup. There were a few exception, but 90% of the time the "spare weapon" was a defensive tool/close work tool. On the latter: you could parry with your main weapon, step very close to the enemy and stab with the dagger.
With the beginning example of dual wield Lances: it will distribute the force of the Impact if both hit at the same time, so you should get half damage dice for that , nah it is obvious that you autofail to hit since you cannot control your horse anymore because you got no Hand free for the reigns.
It is so silly that is where the "ruling not rules" steps in and the DM simply says "nope". No rule set is perfect, GM intervention is a normal part of the game.