Just when they stab each other there was clear eye contact. Danzo could have used it on Sasuke and make him fight Tobi, right?
Danzo had just used the Kotaamatsukami at the Five Kage Summit, and I believe that the eye needed some time to regain the power, that's why he asks his subordinates to fight Tobi first. Then while fighting Sasuke he uses the other Sharingan (located in his arm)... He basically wanted to use the Kotoamatsukami against Tobi, because he was the main guy to be controlled.