Itachi and Tobi both said to Sasuke that they worked together in the Uchiha Massacre. Itachi agreed to help Tobi get his revenge on the clan just as long as Tobi didnt attack Konoha or Sasuke. I know that Itachi put Amaterasu in Sasuke's eye to prevent a meeting between Tobi and Sasuke, but could Itachi have done more. Furthermore, Itachi knew Tobi was in Akatsuki so he had ample opportunity to kill him for the good of Sasuke and Konoha. Like wise, Tobi must have known Itachi was a spy because of the way he cared for the leaf so why didn't Tobi try to kill Itachi?