Guinea Pig help please?

I bought two young guinea pigs just over a week ago, they are both female. They are in a suitable hutch outside. I am getting concerned though because one of them seems to be bullying the other - she bites her alot and chases her out of the closed quarters of the hutch and then stands guard at the entrance , forcing the now intimidated piggy to spend periods of time in the main section. I thought things would settle down but it seems to be getting worse and I feel sorry for the one being pushed around (who incidentally is slightly bigger in size) so to clarify.. the smaller girl is pushing the bigger one around. They are 9 weeks and 12 weeks old. What should I do?