I'm genuinely kind of haunted by "Mirror, Mirror" splicing Uhura's wildly glitzy, swaggering, calculating seduction of Mirror Sulu for tactical purposes immediately after Kirk's sultry, noir-glamour, calculating seduction of Mirror Marlena for tactical purposes.
It's not even just that: in the earlier scene, we're shown Kirk nearly jumping out of his skin when he first sees Marlena in his bed, and remaining visibly uneasy and standoffish for a moment before working himself up into playing along. In the Uhura-Sulu scene, meanwhile, the episode pauses a beat to show Uhura visibly bracing herself before throwing herself into the seduction of Sulu to get their plans into motion.
Later, incidentally, it's Kirk who overpowers Mirror Sulu and Uhura who overpowers Mirror Marlena. It's even Kirk and Uhura's tag-teaming that lets them overpower Mirror Spock; Spock is kicking everyone's asses until Uhura shoves a decoration at Kirk, who immediately bludgeons Spock with it and nearly kills him.
Quick addendum:
Obviously, part of the reason for the doubled scenes are (equal opportunity?) eye candy ft. evil universe shenanigans, make-outs, and loving shots of Nichols' and Shatner's biceps and shadow-sharpened features (and their respective """love""" interests get their shares of this too, with Sulu and Marlena framed as essentially evil sexy pirates). But the show is also extremely consistent in maintaining this thread of fraught connection between Kirk and Uhura around the commodification of their bodies and charisma, whether the episodes in question are fantastic ones like "Mirror, Mirror," comparatively subtle and layered whodunits like "Dagger of the Mind," trash fires like "The Gamesters of Triskelion," or messy ambitious ones like "Plato's Stepchildren."
So I do think there's something very thoughtful and smart about giving each of them that brief but lingering beat to show how fraught and uncomfortable their feelings about this really are, and placing both sequences back to back to reinforce their rapport (in fact, one of the things I appreciate most about TOS is its deep understanding and use of plot structure as well as the literal events happening to indicate meaning). The seductions are profoundly artificial compared to their behavior when actually interested in someone (sometimes the same someone). So while what happens, in-universe, is Kirk flirting with Mirror Marlena (and having to defend himself from her) and Uhura flirting with Mirror Sulu (and having to defend herself from him)—as a story, the structure insists the real, significant connection here is between Kirk and Uhura.
