The Totsuka Blade would need to pierce Eight Branches in order to seal it, of course. But it's not going to be difficult for Itachi to turn his attention to such a thing, especially considering the kind of formation the brothers took up against Kabuto [
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]. They'll be close to eachother at pretty much all times, particularly if Susano'o is being used. And you speak as if Orochimaru will be able to crush Sasuke's Susano'o effortlessly if Itachi does not immediately get involved; this is certainly not the case if you're taking into account the hype which comes with the acquisition of the Eternal Mangekyou Sharingan. Sasuke has his own Amaterasu Blade which is more than capable of keeping the Hydra at bay long enough for Itachi to pierce it [
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]. The only way Kabuto is going to get through Itachi's Susano'o is if he can force him to dematerialise it - he'd have to use Hakugeki, which he would've (at least, if he's IC) already done by that point in the battle. If Itachi is sealing Orochimaru, do you believe Sasuke will stand there in awe, or don't you think he'll actually do something to increase the chances of his team's victory? I don't think it's wise at all for Orochimaru to use Yamata, and i'd even go as far as saying his team would fair better if such a technique remained unused.