I dont kno if anyones said this already, because I cant be bothered to read the replys lol but guinea pigs can often fight wen they r going through puberty, mine did, and it is literally the same phase that teenagers have, so if its only occasionally theres no need to worry but if its all the time then seperate them for a little bit and then try puttin them back together again. Im no expert, but it worked with mine =)