The reason that your guinea pigs chatter their teeth at each other is that they are both males (I am guessing). They wont do it to you 'cause you are not another guinea pig male. Sometimes males will do it if you approach their cage and they think you are entering their space and there are females around - but only some males are that game. One of our guinea pigs is particularily territorial and will make agressive body moves - but this is all show. He is a guinea pig and completely harmless.