Is there a way to reset a person/completely unmerge?

Hi all. I made a mistake and merged my wife and her sister (I will post a a parallel support thread in reddit/relationship-emergencies as well). Any advice on how to fix this? I figure what I want to do is reset my wife's "person" and start over merging just her again. Otherwise I'll probably leave this as it is as I'd rather not reset all faces... Any advice appreciated, thanks!