From a search online of Gide's French text and of an English translation, I was unable to find a page where these two words literally occurred together. However, if one is speaking of the general idea of, or conflict between, morality and immorality in tandem, one could say it occurs all over the book. Michel, despite announcing early in the narrative that he wishes to discard his old concerns inculcated by a moral upbringing and instead to live for the "physical," never fully does so—at least until, arguably, the final sentence of the book, when he admits that he probably wants to be with a boy rather than a woman. He retains a sense of his love for Marceline, while simultaneously mistreating her and even telling her obliquely that he no longer cares about her:
"I understand," she said to me, "I quite understand your doctrine—for now it has become a doctrine. A fine one perhaps," she added sadly, dropping her voice, "but it does away with the weak [elle supprime les faibles]."
"And so it should!" was the answer that burst from me in spite of myself.
If one consults the original, we see Marceline is using vous rather than tu to address her husband, so there is not much intimacy between them. The point about this passage is that it is dominated by Marceline's articulation of a moral point in her criticism of Michel, who then acknowledges his own immorality. But he does so "in spite of myself." This shows that even as he's acknowledging having become an immoralist, he regrets it and only admits it when Marceline brings it out forcefully.
Earlier in the narrative, there is a similar turning-point, except that in this case, the one speaking to Michel is the opposite of Marceline. This is Menalque, who functions similarly to Lord Henry in The Picture of Dorian Gray. Menalque is the "tempter": the one who brings out the tendencies lurking within Michel. Menalque detects something about him:
"The fact is," said he, "there's a 'sense', as people say, a 'sense' which seems to be lacking in you, my dear Michel."
"The 'moral sense'," said I, forcing myself to smile.
Yet why should Michel have to force himself to smile if he truly has no sense of morality? The entire story is a battle within Michel between his inner sense of guilt and his lingering compassion for Marceline on the one hand, and on the other, his decision to overthrow the morality implanted in him by his upbringing and by society.