In every folie à deux there’s a primary individual.
And there is usually a mutual obsession, which is felt much more strongly by one of the parties than the other.
The FBI has done some groundbreaking research on this very dangerous dynamic. It often is initiated by the chance meeting of individuals both of whom harbour an immature and hidden desire for revenge on others.
Their meeting precipitates a sequence of events whereby they cause damage to others, partly as a way of controlling and managing intrusive (sometimes subconscious) feelings of inadequacy and/or shame.
The parties in these relationships are often intelligent people, though harbouring those immaturities and insecurities, and can present well before it all starts fracturing.