No real need for two copies unless you wanted a huge map. In a typical two player game each side will start with 3 or 4 ships. In a four player game each side might reach 6 or 7 ships. Since your orders are limited there is little benefit to having a lot of ships on the board. You would have to modify the game to allow players to have extra orders each turn but in the end I do not think a second copy is worth it.