You can technically accomplish what you want to do but it will require time and a co-operative ship.
If you meet a ship at sea and they are friendly you could have them invite one of the two of you to their ship. You would then invite them to your ship. You would have to become friends and it would be a one time only thing but it would be a fun accomplishment.
A friendly sloop would be ideal as you would each have a friend to beat up lol and new friend to beat up as well.
Good luck!
As for long term. I think private servers are inevitable. They are just getting the game going and the current system was obviously the best way to start and a allow everyone to play but to start with private servers would have just led to exploits. Sail away!