I find it odd that the prosecution would ever have claimed that one person couldn't possibly have done that. I mean, it's their job to find reasons to believe in the guilt of the defendants, but a simple review of murder cases committed with knives should be able to dispense with that. I've only read several, but one of the things that surprised me was the sheer number of times which the perpetrators frequently stabbed the victim. In one case that I read, the murderer stabbed the victim over 90 times.

I suspect that murderers often continue in a panic or frenzy when they discover how hard it is to get their victim to die. An inexperienced knife wielder against a struggling victim can easily stab the victim dozens of times without inflicting a single individually fatal wound.