Mirror Spock forcibly melds with McCoy in the mirrorverse. Not shown in the prime time tv episode is how hot they both find it, coming in their pants as they separate. McCoy's shocked, slightly overwhelmed, but not too distressed cos hey when it's hot it's hot. Mirror Spock is completely obsessed though, and can't leave it alone.
A year or two later, Mirror Spock has found his way into the prime universe and the quite serious bond he and McCoy have pings McCoy. McCoy is not happy about it, but try as he might the bond is there, the man is hot, and he feels better when he's near Mirror Spock. It's frustrating, unhealthy, often frightening. But McCoy keeps secreting his way over to Mirror Spock as often as he can, lacking the agency to stop himself. And Mirror Spock is changing, he admires McCoy too much not to.
As they get older and life changes maybe they'll find a way to live in the open together. Or maybe it's all gonna blow up and be a devastating divorce that's probably the best thing for both of them. Read and find out (but mind the tags, there's some messy shit going on)
This fic was part of the @sponeszine NSFW mirror verse issue! Check it out, there's some beaut stories and art in the zine 💙