Correlative conjunctions consist of two parts, both of which must be included in the sentence. Some common correlative conjunctions are both . . . and, either . . . or, neither . . . nor, not only . . . but, and whether . . . or. Like coordinating conjunctions, these conjunctions may link words, phrases, or clauses. The units being joined by a correlative conjunction must be parallel in grammatical structure and importance.
Marasmus is the condition that results when a child's diet lacks
both total calories and protein.
--"Metabolic and Deficiency Diseases," Compton's Encyclopedia
If you use a correlative conjunction to join two independent clauses, separate the clauses with a comma before the second element of the conjunction.
Either Jan will conclude the experiment by February, or she will ask for additional research funds.