Do you play indoor with both of them? And do they always act like that? I mean, if everything was fine and they've just started attacking each other I hope it's not a sign of a disease. I always check my dog's behavior and if I see any changes I browse forums like this one to be sure if it's not something bad. Recently my dog was licking her paw all day long and I thought she's got a splinter, but couldn't find it. I googled the information and turned out that the reason why dogs are licking their paws may be a shampoo allergy. So we changed it and luckily everything is fine now.