This achievement like many other require a bit of luck. Start as Castile and get a royal marriage with Aragon which already have a Union with Naples.
With some luck the Aragon King will die without an heir and you will inherit both countries and earn this. if your not afraid of some bad reputation you can of course try to claim the Aragon throne before that happens. That would require you to win a War which can be tricky depending on how the alliance was setup at the beginning of the game.