Given that a server has a maximum ship count of six (to the best of my knowledge), then to bring two ships would essentially mean that you and your friends would make up a third of the entire server, instead of a sixth.
Which sounds a lot more significant than ‘one less ship to encounter’
It is also just overpowered. Part of the balance of allowing four players to work together is putting them on a more cumbersome ship with a reliance on teamwork to be effective. The sloop can be comfortably operated with one player, so having four people across two sloops working in tandem sounds like a monster combination, and I imagine we’d see significantly fewer galleons on the sea.