I would regard the use of "also" as (stylistic) pleonasm, even if it is well-attested and therefore, by definition, "grammatical".
Not only should you be able to speak but (be able) to write.
You should be able not only to speak but to write.
Your example
This {thing} allows you not only to maintain a uniform visual style, but also allows you to introduce new developers to your codebase.
doesn't gain much by the "not only x but y" pattern; in fact, I think it loses clarity as a result of it. The relationship of the two ideas (uniform visual style, introducing new developers) is not immediately apparent. I'd make two sentences out of it, and expand on how new developers can be more readily introduced because of the uniformity of visual style.