In previous installments all of my examples of fictional psycho-killers have been singular. However, it's possible to have more than one psycho-killer in a given narrative. Again using films as an easy resource, "dual psychos" are a favorite device in the realm of the "fake psycho" narrative. For instance, 1964's BLOOD AND BLACK LACE, which numerous critics deem the first of the Italian "giallo films," has two people donning the mask of the killer at different times during the story.
Thereafter, the dual-psycho film became common, ranging from 1971's BAY OF BLOOD to 1996's SCREAM. However, it's relatively rare to encounter two psycho-killers who are not initially associated with one another, and when this trope is used, it tends to yield comic results, as with the 1987 film PSYCHOS IN LOVE.
The comedy may stem from violating the "one gimme" premise associated with most films in the subgenre. However, this may tend to be more true of films in either the naturalistic or uncanny domains, given that FREDDY VS. JASON works as "straight" horror despite its use of assorted comic touches.
However, marvelous psychos don't seem to lend themselves to the concept of the extended family. The most famous iteration of the "weird family" trope arises from the American "old dark house" film, implicitly named for the 1932 film THE OLD DARK HOUSE.
However, though the HOUSE is full of weirdos, only one of them is intentionally murderous, and his status as a "psycho killer" is debatable. 1965's SPIDER BABY is probably a better exemplar of a family of psycho killers.
Of course, the first two TEXAS CHAINSAW MASSACRE films qualify, even though Leatherface remains the primary psycho.
Rustic environments seem to breed families or even whole societies of psychos. The 1972 DELIVERANCE presents a whole society of degenerative hillfolk, though the film remains firmly within the naturalistic domain, unlike the more overtly weird WRONG TURN "killbilly" franchise. However, the largest psycho-killer society is almost certainly the town of hostile Southerners described by H.G. Lewis's film-title, TWO THOUSAND MANIACS!
One last note on the psycho-killer narrative is that it has proved so popular as to spawn hybrids that don't quite conform to my model. The first four films in the LEPRECHAUN series are almost indistinguishable in tone from the ELM STREET series, in that both concern a hideous supernatural being who kills indiscriminately and makes many bad jokes. However, the Leprechaun is "outside horror," in that he has no psychological motives; he's just evil.
There are even a few serial killers who make pacts with the devil for their marvelous powers, such as the Cenobites of the HELLRAISER film-franchise. But in this and similar cases, it's understood that whether the modern-day killer calls upon Old Scratch or Leviathan, his powers are rooted in a tradition of folkloric magic outside the province of the cruder, but more normative, psycho-killer narrative.