When and Where do u learn he double strike trait

my samaraai has that trait. i dont know if its a class thing, but i had to pay the guild (bushi i think) for the training. i think your character has to be a certain level also.

I have this trait with a Barbarian with this also, but he's like 4'rth or 5'th level in the guild. hope this helps. you do need to buy it, then learn second weapon.