This is where the law fails in this instance. It's my understanding that the law permits a reduced charge for multiple people when a) one or more of them has definitely committed murder and b) there is no evidence to demonstrate which. So, if two people go into a room in which a third is murdered then, in the absence of further evidence, neither can be found guilty of murder. What should be the case is that all should be charged with murder, with additional time being given for their non-cooperation.
In the absence of harsher punishments that is exactly what I'd mandate. I can't think of a reason not to do so. Of course, proper sentencing would make the issue moot (or so you'd think - it was reported only yesterday that an inmate in a UK prison got a guard pregnant).
They were both charged with murder and were both acquitted.
You seem to be implying that when it isn't possible to prove who committed a murder, juries should just convict everyone who might have done it.