You mean there is someone posting under two handles?
Or do you mean two people posting under the same handle?
There are other alternatives, y'know. Two collaborators are posting using different names; or maybe one person is adopting different identities when posting about different subjects.
Then again, does it really make any difference? Unless the topic is controversial and the doppelganger is used to create an illusion of agreement to intimidate some third party, it really doesn't matter. We are all, after all, hiding behind our handles.