There is a good chance they are just fat then if they are definately female and havn't been near a male. How old are they?
Reduce the number of pellets and veg they are getting and they should become nice slim piggies.
Saying this, one of my piggies was quite fat in comparison to my other four... we thought it could be ovarian cysts... she had an ultrasound that didnt show anything. This piggy has now gone to the bridge from an unrelated illness though, so we didnt know if she did have them or not.
My point is... this is another reason for unexplained 'fatness' around the rear of a guinea pig.... I really dont think this is what is happening in your case though as they are both fat which suggests it is food related.