Folie-a-deux means

This is an uncommon delusional disorder characterised by a sharing of delusions between usually two ( folie-a-deux) or occasionally more persons ( folie-a-trios, folie-a-quatre, Folie-a-Famille), who usually have a closely knit emotional bond. Only one person usually has authentic delusions due to an underlying psychiatric disorder, most often schizophrenia or delusional disorder. Ref: A Sho Textbook of Psychiatry, Niraj Ahuja, 7th Edition, pg. no. 85