I don't think folie à deux is exactly the phrase I want here, but does anyone else get the sense that without one another, neither X nor Y would have progressed from fantasy to actual murder?
Partly, I don't think either would've murdered on their own (if, of course, either is found guilty of murder here), but I do wonder if one or both might've done if they were to find someone else of similar mindset.
X may be claiming all of her messages were fantasy (and clearly some are given the police could find no evidence of the Laydon stuff), but those are very very dark fantasies, with an awful lot of planning, and when the one with the most planning ends up happening in much the same way as the plan says, it's hard to think of her as anything other than dangerous. Y may perhaps be more led than leader, but has enough interest in violence and torture to likely end up in contact with someone else of similar interests too.