Board Thread:Versus Debates/@comment-26005711-20151018173351/@comment-734582-20151027080437

LoneNinja wrote: So even if he is limited to one person at a time, he can use the first to make Itachi Surrender or w.e he wants to do then do the same to Madara. Madara and Itachi know Kotoamatsukami and fight Sishui from both sides, so he only ever has one in sight. He uses Koto. Madara or Itachi immediately notice and fires a Genjutsu at the other, breaking Kotoamatsukami. Repeat once more and you're safe for the rest of the fight.

It's literally how Itachi and Sasuke broke genjutsu from Kabuto and it's made clear that Kotoamatsukami is easily detected by a third party.