You know a lot of 14 year old girls who double up their workouts? Define a lot. Come to think of it, how old are you and why do you know "a lot" of 14 year old girls?
I may come off as flip but I am curious as to how many 9th grade girls are doing doubles. My kid likes to run but there is no way she is getting up at 530am and running by herself in the dark and she is more obsessed than most of the other girls on her team.
The typical mileage I have heard of locally for freshman girls is 35-40 miles/week. Why would they double up before their junior or senior year?