When using a correlative conjunction, both clauses have to be parallel. He’s not only funny but also intelligent. He has not only a sense of humor but also intelligence. Matthew is going not only to Egypt for a month but also to Greece for a couple of weeks.
Parallelism and How to Use Not Only…But Also in a Sentence
When using not only . . . but also in a sentence, parallelism should be the goal. But what does this mean, exactly? It means that the words following both parts of this correlative conjunction (i.e., not only and but also) should belong to the same parts of speech. For example, if a verb follows not only, then a verb should also follow but also. Using different parts of speech after each part of this correlative conjunction makes the sentence imbalanced and, frankly, just awkward. Consider the examples below:
Both sentences mean the same thing, but the first is imbalanced. The adjective intelligent follows not only, whereas the dependent phrase has a great sense of humor follows but also. In the second sentence, however, the adjectives intelligent and funny follow not only and but also, ensuring that the sentence is parallel.
Comma with Not Only . . . But Also
Should you use a comma with not only . . . but also? Not usually. Generally speaking, a comma should not be used to separate pairs of conjunctions in a correlative conjunction. As you may know, however, there are often exceptions to grammatical rules, especially when dealing with commas.
It’s correct to write the sentence this way, with no comma:
But if you really want to show special emphasis, you can add a comma:
Have you ever thought you needed more balance in your life? These days, people talk a lot about achieving balance, whether it be moderating their diet or dividing their time between work and home. This concept of balance is not a new one, though. In fact, you can see the human inclination towards balance in most academic subjects including math, science, and yes—grammar! The usage of not only . . . but also is a good example. When using this correlative conjunction, it’s important to keep the notion of balance in mind; from a grammatical perspective, we call this idea parallelism.
from Grammarly Blog https://www.grammarly.com/blog/parallelism-with-not-only-but-also/