Consider if a process was a member of two control groups, one of which has a memory limit of 200MB and one of which has a memory limit of 1G. To which control group should that process's memory be accounted? Half each? In proportion to the memory limits? In proportion to the number of processes in each? Something else?
What happens if one of the two control groups is at its limit, but the other is not - is the process prevented from allocating more memory? If not, how does its memory allocated get accounted now?