Partners
When the word partners comes to mind i think of two people, such as Ian Brady and Myra Hindly.  However lets not forget that just as many cultures can have marriages with several partners so can the serial killer.  Though it is rare, the serial killer sometimes comes across those of his or her type, or comes across someone whom he or she has so much power over that they also become a criminal, and not just any criminal.  A serial killer.
The Lonely Hearts Killers
The Manson Family
William Burke and William Hare
The Moors Murderers
The Hillside Stranglers
The Zebra Killings
Chicago Rippers