Guinea Pigs prefer to live in pairs and love company. This is because of their naturally sociable nature. If they are not in pairs or groups they may develop abnormal behaviors and be very lonely. Two girls or two boys are best, unless you would like to have extra guinea pigs.
The best way to do this is to keep them with their own litter, that has been raised together. This would mean that they are used to each other and there wont be much fighting.
It can sometimes be difficult introducing a new guinea pig. You have to introduce them when they are young. Sometimes it can take a little while for them to become friendly, but eventually most guinea pigs will live happily together.