bjlillo nailed it, you need 14 to get the second.
Getting a second navigator is pretty tough, because it requires controlling a lot of sea regions. But since there are only two navigators available in the game, if you do pull it off, you're effectively preventing your opponent from getting any navigators, and forcing him to get points from other things, so it can be a good move if it dovetails with the rest of your plans.
Great game BTW, needs more love!