Sacrificing a senior captain to make another captain stronger is a ridiculously dumb war tactic and very bad writing. It makes Kenpachi absolutely untolerable. Unohana deserves way better
The only way it doesn't really make sense is if the two captains as they are are getting the job done, which they weren't. It's common anime logic. If the two separate aren't effective, but the two "combined"' are > the two separate, then you obviously make the sacrifice.
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Seplito Nash, Smelling Like the Vault since 1996 Step FOUR! Get Paid!