Here is a multiple-choice question:
_____ not only I but also he going to Paris?
Which choice is correct?
This is a tough question indeed. There are two good choices: am and are.
The problem comes with picking the word that fits with/links to I and he in that order.
This is not the correct answer but can be debated. When including yourself in a group, you put yourself last: he and I. Since this is first-person plural, you use are. However, since the sentence puts I first, the previous rule is broken so are I and he won't work.
This is the correct answer. When referencing yourself, you say I am. The sentence puts I first instead of he, which allows for this rule.
The reason the am rule is followed and not the are rule is because the structure of the sentence (but also) splits I and he into different parts, thus breaking the are rule.
Do we answer the question "who's there" with "only I" or "it is I"? It may be grammatical but it sounds absurdly artificial and must have done so for a century or more. "It's only me" in the third person is what sounds natural. The same with your problem construction. "Am not only I..." is what confounds the rest of the sentence. The idiomatic solution would be to make you and him both into third persons: "Is it not only me but also him going to Paris?"