I think that instead of having the 2 person ship being the same as the single, make the 2 person a bit longer, add another cannon to each side, and maybe another sail. Not as big as the galleon, but a decent size larger than the single.
The Barque would fit the bill for what you suggest. It is a small 3 mast ship. Very popular with pirates because of its size and decent firepower. I agree, there needs to be an in between ship for 2 player that is better than the sloop.
