Which is correct:
Myrtle’s party took place in her and Tom’s apartment
Myrtle’s party took place in Tom's and her apartment
I know that this construction always appears slightly clunky; when I'm writing "x and y's" I always use the order of subject-first (so your sentence would read "Myrtle's party took place in her and Tom's apartment"). I wouldn't by default utilise brackets to separate 'her' and 'Tom'; it might be preferable to also introduce the second subject ("Tom") in a previous sentence in order for you to simply write "their apartment".