‘Folie à deux’ is a psychotic disorder where people share the same delusional beliefs and go insane together. It often occurs when two people have a strong connection, are socially or physically isolated, and at least one of them has a history of mental health problems. Though it does happen in sibling or parent-child relationships, the disorder is most common in couples – and in most cases the only treatment needed is time apart. Source Source 2 Source 3