"Both" takes a plural noun -- it means "all two". Your sentence should read:
Both posts have been replied to.
(Note that the preposition "to" is necessary here.)
It should be the passive voice because the posts themselves do not reply: people reply to posts. In the active voice, the sentence would look like this:
[People] have replied to both posts.
The identity of the subject, though, is not important, so we can leave it out. When we do, we have to use the passive voice. "Both posts" becomes the grammatical subject of the sentence (although not the subject of the action described by the verb) and moves to the beginning of the sentence, leaving the preposition "to" at the end.
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