folie à deux
noun
fo·lie à deux fȯ-lē-ä-ˈdœ 
ˌfä-lē-ˌä-ˈdər,
-ˈdə : the presence of the same or similar delusional ideas in two persons closely associated with one another
Example Sentences
Recent Examples on the Web There's a moment late in the play when Hinkypunk's Solange gets a glimpse beyond her folie a deux, recognizes the inevitability of what's to come next for her and her little sister, and gives in to it. 
Tony Adler, Chicago Reader, 27 June 2018 The term folie a deux refers to two individuals who plan and commit crimes that neither would concoct on their own — the basis for (fact-based) movies like Hitchcock’s Rope, or Peter Jackson’s Heavenly Creatures. Gary Thompson, Philly.com, 7 Mar. 2018
Tony Adler, Chicago Reader, 27 June 2018 The term folie a deux refers to two individuals who plan and commit crimes that neither would concoct on their own — the basis for (fact-based) movies like Hitchcock’s Rope, or Peter Jackson’s Heavenly Creatures. Gary Thompson, Philly.com, 7 Mar. 2018 Word History
Etymology
French, literally, double madness
First Known Use
circa 1892, in the meaning defined above
Medical Definition
folie à deux
noun
-(ˌ)ä-ˈdœ̅, -ˈdə
plural folies à deux
ˌfō-ˈlē(z)-(ˌ)ä-
: the presence of the same or similar delusional ideas in two persons closely associated with one another