Don’t do the Darth Maul thing, the Darth Maul thing only works because lightsabers don’t get caught on anything and you can shut it down. Given that your character is going to be fighting in such tight quarters on a ship or in a siege, he’d be just as likely to stab his own soldiers or friends as he would the enemy. He would also have a great deal of difficulty pulling the blades back out of the enemy. Staff weapons require a fairly large amount of space in order to be wielded effectively and because the blades are so long and the hilts so short, he would have difficulty bracing effectively or using spear tactics.
A pirate does not have a lot of space in which to wield their weapons. This is why single edged weapons such as the cutlass and the saber were the weapon of choice aboard ships, there was less chance of the weapon getting knocked back into your own chest if someone overpowered you or in the flurry of excitement of boarding an enemy ship. There were better weapons available at the time than the cutlass or the saber, but they were a greater risk to wield in such extremely tight quarters.
This weapon of his would also have to be specially made by a blacksmith and that’s a fairly large expense for a weapon that’s likely to get him keelhauled by his buddies.
Give him a normal cutlass, a pistol, and a rifle with a bayonet. Do some research on the weapons of the time. While the gunsword was not an effective weapon, it was a rather ingenious attempt at one. It pops up during the Golden Age of Piracy. So, if you really need to give him a “special” weapon, give him that one. It’s at least historically accurate to what people were using. Yes, it is ludicrous but it’s real.