I do fencing and Renaissance fencing, following old italian swordplay style formulated by Achille Marozzo that, guess what, is doubled.
The facts are:
- Actual fighting isn't a fair exchange of hits. If my "entrata" (attack intent) is good enough, you don't do any "risposta" (attack on your round). The goal is to lock you in defence and get increasing advantage until I touch. It's unfair. That's mean you cannot exactly replicate it by a d20 system without making players mumble.
- The "seconda mano" (secondary weapon) isn't made just to parry. Many of maneuvers are made to hit with the "prima mano" (main weapon), get parried or move the opponent where you want, damage with seconda mano. So, the second weapon is made to hit. But any combination is legit and having a varied set of maneuvers is what make the style efficient.
- Once you touch, you stop. Piercing something with both blades mean you're stretched without any defence left. You dunno if the opponent retaliate or there are other duelists near.
It's quite impossible to replicate the thing with a fair engine that allow for HP, doesn't consider pain, stretch, body physics, and give scarce weight to fighting proficiency.
So, to me, giving another attack with a penalty (an
heavy one hopefully) is a correct broad abstraction. It's just harder to parry twice in a row = more chances to deal a blow.
Probably what your player is thinking is the hero swinging his weapons at the same time. So two axes = two axes swinging each attack. You can kill his dreams by telling him to wield two pens and show you how he'll chop a tree this way (he can't).
But a more detailed game, maybe GURPS, might give different weapons advantages against opponents in different armor, or against opponents at slightly different ranges.
GURPS
do consider my points, you have a -4 penalty after the defence action. I'll attack, you parry, I'll attack again, you parry at -4. If I touch, the damage is again a penalty to your next action, leading to lock you in defence and in a death spiral that's exactly how you win a fight.
De facto, the most skilled duelist is the horse to bet to. Next to the stronger one. The "lucky one" doesn't survive for long.
Dual-wielding makes more sense to me in armor-less situations, like boxing. If any old slash is dangerous, why not make a lot of them?
My style use armor. Helmet and light padded armor, that was the renaissance swordplayer setup on fields. We practice with fencing masks and football protections.
You get the point: IRL, you want to make
a lot of slash, but the moment you do a bad slash, you get pierced and you die. A martial style teach you how to deal the larger number of slashes without losing composture. Is the same as boxing, it's not a rumble with a flurry of fists. But the moment an exchange begin, a lot of punches are delivered in short time. Each punch try to connect but doesn't leave you totally offguard. This is maxed out in oriental martial arts, where each attack maneuver contain a defence stance too.
IRL, you can do a flurry of attacks, but the moment you face a more skilled opponent, he laugh at you: style is about how to kill the "flurry attacks" guy.