It’s not really doable to wield two at the same time and a military-esque trained pirate (like a former officer of the British Navy) would most likely be wielding a saber instead of a cutlass. You can dual wield in fencing, but that’s using a long dagger and the second weapon is meant for defense. You can also fence with a small shield. Dual wielding swords, unless they are short swords like the butterfly knives/swords of Wing Chun, is a bad idea because the blades get in the way. Unlike escrima “sticks”, where you don’t harm the weapons by banging them into each other during early training, a swordsman is more likely to harm and be hampered by the length of the weapons when striking, the weight of his weapon in his offhand, and is at risk for destroying them when he clangs them together. Your character will have better speed, dexterity, and striking power with just one.
Edward Kenway fights with two sabers because the game is, I guess, trying to make a point that he doesn’t really know what he’s doing and that shows in his style when compared to Ezio or Altair. However, your military-esque pirate would probably carry four pistols and those would be more useful to him. Pirates carrying multiple pistols was also a real, historical fact. Edward’s fighting style is actually hampered by the fact he’s carrying two. The Assassin’s Creed IV doesn’t make a point of that though. You will notice, though, that Edward commonly uses the offhand weapon for defense.
There’s a bad habit in Hollywood and games where it’s believed that two weapons used together equals more skill or more offense. I personally blame D&D, but the second free hand is important for balance, used for distraction, has more dexterity, and creates better openings than a second blade. Also, and this is important, your pirate would not carry two swords. Why would he carry two swords when he could carry a pistol and a sword at the same time?
This is important logic.