[AntiGuv]

I don't think there's any concrete evidence here that there are two people involved. However, it's certainly not unheard of - the Columbine murders are the most recent that comes to mind. All it means is two sociopaths managed to hook up with one dominant over the other. In particular if one of the two is a woman, then that especially doesn't indicate anything whatsoever in and of itself. A great number of serial killers have had accomplices - most notoriously Randy Kraft in California (although they never caught the accomplice). The Green River Killer was probably a duo as well. There was another pair in the Midwest (I think Ohio) whose names I can't remember now and another pair that travelled the country. Here's a good psychology treatment of the "team killer" mentality that I read some while ago:

http://www.crimelibrary.com/serial_killers/partners/team_male/

They discuss another four pairs of serial killers here as well.

[john in missouri]

What a depressing read. I think it's time to shut down the web browser and put on some jazz.

[El Gato]

I don't think there's any concrete evidence here that there are two people involved

Of course there's not much concrete evidence period. There is at least some eyewitness evidence that there are two people involved. It's probably not very reliable, but the same sort of evidence as that indicating "white box truck", "white mini-van"