"not only... but also" is a correlative conjunction. Correlative conjunctions are those that connect two equal grammatical items. When using these conjunctions, it is very important to make sure that we maintain verb and pronoun agreement. We also have to be careful about maintaining parallelism.
There is more than one conjunction in the sentence, the word but acts as a coordinating conjunction. Also the words not only and but also act as a correlative conjunction.
