Folie à deux. Essentially, a psychosis shared simultaneously by two people. I'm sure I'd heard the phrase more than once in my life, but most recently, and specifically, heard it attached to Ursula and Sabine Eriksson (here's the Wikipedia, but here's the Cracked article where I first learned about them).
What causes somebody to "go crazy" (obviously, "crazy" isn't really the most PC to refer to this, nor the most effectively with regards to expressing diagnostics)? Granted, we in the business may call something like that a psychotic break, and as a total layperson who knows next to nothing about the case, I won't call it a schizophrenic break, as they appear to be free-range at this point and nobody in fact seems to bring up schizophrenia, that I've seen (or that I remember, anyway). But both twins? At the same time? That's some road trip.
You can watch the documentary about the Erikssons, Madness in the Fast Lane, at Documentary Storm, which has a number of free documentaries available to watch streaming (so you're aware, if you do watch Madness in the Fast Lane, they play the footage of the women getting hit by cars on the freeway repeatedly. Spoilers: they survive.). I don't remember liking documentaries much in the past, or maybe I didn't really think about it. But I like them quite a lot now. Maybe they've gotten better? Maybe I'm just a grown up? Whatever.
But, introduce a notion like folie à deux into a consciousness like mine, and I wonder two things: how many "freak occurrences" can be attributed to it, and how can I use it in a story? Is it useful in the case of establishing an unreliable narrator? Or is that too much? Just something to add to the subconscious stew.