By double mangekyo, he is referring to both his left mangekyo eye and his right mangekyo eye. We know that Tsukuyomi is used with one eye and amaterazu is used with another. Susanoo is used with both. We know that three eye techniques are awoken within the regular Uchiha with the mangekyo sharingan. I hope that this explains things. I am sorry that I am too lazy to post the chapter. I will if I have to though.
