What does this guinea pig behaviour mean?

That is common guinea pig talk for setting up the rules of the cage, they understand each other quite well. The bigger guinea pig is just letting it be know he intends to be the boss of the cage, as long as there is not fighting that draws blood they are working everything out in their own silly little way.However, if blood draw does happen you will need to separate them at once as they will fight until one of they is seriously hurt. Just because they were brothers does not matter, even if they had not been separated from birth they would still work out who is going to be boss of the cage. As long as they have a nice size cage so each has plenty of room every thing should work out well.