Aggression is usually a problem only when boars or sows are housed in same-sex partnerships (although infrequent with a pair of sows it does happen). It is therefore possible that you have the sexes wrong (see the link below for help).

One way to try and minimize the problem is to smear a little mentholated vapor rub around the rump and the chin of the pigs. This takes away the body odor differences between the guinea pigs, and can decrease the conflict. It is also important to provide plenty of food and if necessary have two or more bowls of dry mix to try and minimize any disputes over feeding space.