Well there is definatly a difference in fighting and playing.
They could definatly be fighting, trying to be the alpha dog.
They need a "pack" leader, so they are fighting over who gets to be the leader. Start now, and show them that YOU are the boss! I had this issue with my 2 Chi's. I could definatly tell they were fighting. So I just had to gain control! At 8 weeks old, they are prolly just playing, but if YOU think it is getting out of control, then you need to put them in spearate kennels when they being fighting, tell the NO! and keep them in there for 10 minutes...no more or less. Do this every time, and they should quit after a few times in the kennel.
My Chi's still fight every once in a while but over jealousy issues. If one is sitting on my lap, and the other wants attention, then they will snap at eachother.
It is very rare for them to get into it anymore, but if they do, I do the kennel thing once and everything is fine.