It's true Sasuke was present when Itachi fought Kabuto but Itachi had to save his little bro two times, because he was lame a bit...
He had to save Naruto and Killer B as well when they fought Nagato...
And both times his shot was the last...
So, both fights were led by Itachi in the end...
Itachi only saved Sasuke once, the second time Itachi was Kabuto's target and wasn't trying to hurt Sasuke (for the reasons Itachi explained just before). Sasuke then saved Itachi from Kabuto after he was impaled, then when they were put into genjutsu they saved each other, then Sasuke saved Itachi again after he was chopped in half.
Sasuke's involvement, aside from saving Itachi, was essential for getting Izanami to be successful. He helped Itachi set up AND finish the loop. It was a team effort.
Right, when THEY fought Nagato a.k.a. Kabuto controlling Nagato and has limited knowledge of his tenseis' abilities, Itachi was hiding the whole time and watching. Kabuto blasted everyone with Shinra Tensei, Bee recovered and counter attacked, but Kabuto absorbed his chakra and tossed him aside. He then captured Naruto and tried to rip out his soul, Bee intervened with taijutsu but was captured and almost killed, and Kabuto just sort of forgot that Itachi was even there. Itachi then threw kunais at the summons and Kabuto seemed entirely unaware of the ambush even though he had shared vision, then Itachi pounced and saved them. Then Kabuto used Chibaku Tensei, at which point Itachi needed Bee's and Naruto's help to stop it and save himself. Then Itachi ambushed him again, a crippled Nagato, who's Shinra Tensei was recharging. Kabuto even said it was Kabuto's lack of mobility which was his downfall.
Both fights had Itachi being unable to do it alone and learning to trust in the strength of others, something he didn't do while alive.
P.S. Edo Itachi was also FAR more powerful than living Itachi could ever hope of being, and is thus not an accurate reflection of living Itachi.
