as a nurse erik erikson is one of my favorite theorists of psychosocial development that I use to guide my own nursing practice
http://www.simplypsychology.org/Erik-Erikson.html
at any time in the scale you can be "held back" by issues in your life. That being said, "identity vs role confusion" is where these two would normally fall in this scale.
Also in group situations there is usually a "leader" and based on what I have read on this case so far the two seemed to feed into one another based on their magical thinking . I am sure that one of the two thought this fictitious character was "real" and convinced the other one. As to the organic nature of why this happened.. I am sure this will come about after the proper legal documents are filed to get them both psychologically evaluated (I am sure this will happen imhoo :twocents
With all the above being said this will not be the first or the last (sadly said) child that is homicidal.
All the above is just my personal opinion as a nurse.
I truly do hope that both girls will be evaluated and I am so very glad that the girl that they stabbed did not die. That truly cannot be understated imhoo.