Complex sentence: No sooner than they're left in private, however, do they both shift into their surly attitudes and begin exchanging sardonic remarks between each other; in these moments, it's Jim who's the more harsher of the two.
Split into simpler sentences: No sooner than they're left in their private, however, do they both shift into their surly attitudes and begin exchanging sardonic remarks between each other. In these moments, it's Jim who's the more harsher of the two.