Correlative Conjunctions

The fundamental of Conjunctions are that they connect related pieces, clauses or phrases to form complete meaningful sentences. Look at the parts of the speech that try to convey something. Nouns/pronouns and verbs thus take the top most priority. They are the subject, object and predicate of the sentence. Adjectives/adverbs are merely modifiers/intensifiers and stuff. Their presence enhances but they do not almost always form the subject, object and predicate. Prepositions connect verbs to nouns, other than that, their role is devoid. Finally, we are left with Conjunctions because, Interjections are out-of-syllabus totally!!!!! (kidding -- knowing interjections can be quite helpful with exclamatory sentences).

When one or more subjects or objects or actions done by either of them are being connected, conjunctions come into play.

1. I visited not only Berlin, but also Paris - Correct because we are connecting two nouns ( Berlin, Paris) with the same verb visited.
2. I not only visited Berlin, but also flew over Paris - Correct because we are connecting two different verbs(visited, flew over) done with two nouns(Berlin, Paris) to form a complete meaningful sentence, literally just forming a pair!!!!
3. I not only have done this, but also have done that - Correct because connecting verbs(doing/ have done form) with two things(this and that).
4. I have done not only this, but also that - Correct and you try the explanation yourselves!!!!

So you see, the emphasis is on the placement of the first one of the correlated word-pair.