Correlative pairs such as either…or, neither…nor, not only…but also, and whether…or also require parallelism. When you see one of these pairs in a sentence, check to make sure that the words or groups of words immediately following each conjunction are in the same form.
Consider the following sentence:
Either I will attend the show, or they will be attending.
This sentence uses the correlative pair “either…or” to present a set of two options. Are both in the same form? Compare the structures immediately following each conjunction:
(Either) I will attend: pronoun + future-tense verb
(or) they will be attending: pronoun + future-progressive-tense verb
Both constructions use a pronoun followed by a verb, but the verbs do not match. Parallelism dictates that both verbs must be in the same form:
Either I will attend the show, or they will.
Either I will be attending the show, or they will (be attending).
The first version has two future tense verbs, while the second version has two future progressive tense verbs. Both tenses are appropriate for describing an event of some duration that will take place sometime in the future.
Incorrect: Either I will attend the show, or they will be attending.
Correct: Either I will attend the show, or they will.
Correct: Either I will be attending the show, or they will (be attending).
Both latter versions are correct.