No Anatomical Changes Needed
A lot of sources say that duel wielding is not historically accurate, or practical, or even doable, but this is simply not true. What is true is that duel wielding was not historically useful on a battlefield, but from the gladiatorial fights of ancient Rome to the Judicial duels of the Medieval and Renaissance periods, it was not uncommon for masters of one-on-one combat to choose duel wielded weapons.
On a battlefield where combat effectiveness is determined by how well you synergize with the guys next to you, shields and polearms are king. When one guy fights with a spear and shield, he has to work extra hard to keep you in his threat area. But when a row of of people fight shoulder to shoulder with spears and shields, assaulting said formation without either having your own shield or even longer spear is pretty much suicide. This does not leave a whole lot of room for duel wielding.
However, in a one-on-one fight, a shield (without other shields to interlock with) can be readily turned aside with one weapon while you strike with the other. Or a long polearm can be parried (if done right) with one weapon as you strike with the second.
As for Adventurers
If you are creating a fantasy/adventurer type setting, duel wielding actually makes a lot of sense. An adventurer has to do a lot of traveling, often into wilderness with minimal logistical backing, and not enough companions to create a proper battlefield formation. Carrying shields and polearms in the wilderness is cumbersome because they are heavy and catch on branches, and they keep you from using your hands to manipulate your environment. In contrast, a pair of short swords or a side-sword and dagger are very light weight and can be sheathed much more out of the way.
Or if your character is just a guy who hangs out in the woods a lot, it is likely he has a small hand axe, a machete, and/or a general purpose knife. So, with nothing else on your person, drawing a hand-axe and machete to protect yourself with would not be unlikely. If you are attacked by a wild beast you can use one weapon to maintain distance, and the other to inflict harm.
Also, many military swords had very similar blade geometry to a machete and may have very well doubled as wilderness tools. Seax, kopis, falchions, messers, and bowies just to name a few. So, if you want to kind of combine a proper solider with a woodsman into a sort of official Ranger or wilderness scout type of person, then giving him a falchion and hand-axe to fight with makes a lot of sense.
So, if this is for an RPG for example, you can use the weapons and gear as a basis for modifying how well adventurers can travel through and even fight in certain areas. Because a duel wielding class never carries long heavy weapons, he will generally be able to travel and fight better in the wilderness than someone with one big weapon. And if one of his weapons give you a tool advantage as well, then all the better.
Actual Combat Mechanics
Wield two weapons, both with the same effectiveness as a skilled swordsman can wield just one sword
You can not do this the way you are imagining. Duel wielding two swords perfectly is not about being as good as two skilled swordsmen. To fight with both weapons as though they were your main weapon does you no good because a skilled swordsman attacks using his whole body. He plants his feet, turns his core, and shifts his weight as he attacks. So, he can not commit his whole body to two slightly asynchronous attacks.
But there are other ways he can use two weapons together to increase his fighting ability well beyond just having one weapon. With a sword and shield, you have the ability to attack with one side and block on the other. With two weapons you can parry or bind on either side of your body while striking with the other giving you way more options to open up an attack. You also have way better visibility of your foe than when using a shield allowing you to be more proactive. And if you are smart about it, you are wielding 2 different weapons with different strengths and weaknesses. When you look at Italian Renaissance swordsmen for example, you often see a side-sword/dagger combo. This is because the longer side-sword gives you reach, but if your enemy tries to rush you and gets too close for your sword to work, you can instead stab him with your shorter dagger. Another example could be the use of a sicca/gladius combo. In some cases, a dimachaeri could use a curved sword to hook an enemy's shield and then the straight sword would be better at exploiting the gap.
All this considered, if you want to make a realistic duel wielding mechanic for a game, you should not get extra attacks, you should get easier and/or more powerful attacks. The simplest way to do it is to role both attacks and keep the better attack, or depending on how defense works in your system, the first attack maybe drops your opponent's defense score so that your second attack is much easier to land and does more damage.
striking with both blades is akin to punching with both fists
boxers fight using both hands, but you never see them hitting with both of them at the same time, do you? Dual wielding is not about hitting with two weapons; it's about using two tools as two parts for the same purpose; you could use one weapon to open the enemy defense and the other to strike, or one weapons for mid-range and the other for close-range. $\endgroup$