What is the "not only ... but also" sentence pattern called?
The basic pattern is called coordination or conjunction. It's where two distinct words or clauses are being joined in a single sentence (to compare, contrast, etc.)
There's also a class of words called conjunctions (including, for example, and, but, for, or, nor, yet, and so) which are commonly used in sentences with this pattern.
This pattern together with some others that you can find here is called correlative conjunction.