How do you get the two become one keyblade in kingdom hearts 2?

Unless you have a special modification system like a GameShark, you're going to have to use a drive form. There isn't a way to become dual-wielding in Kingdom Hearts 2 besides Valor, Master and Final Form. You get Valor Form towards the beginning of the game when you take the train from Twilight Town to the castle of Yen Sid. Go into the room with the three fairies and you receive new clothes and a 'Drive Form'.