Why dont my two puppies get along?

i have a rottweilier german sheppard mix aged 12 weeks and a west highland terrier ages 13 weeks. And they dont really seem to get along. They sometimes play tug of war with eachother using their toys. But other then that it is nothing more then a bark fest and lots of fighting. Nothing serious though. Neither one is getting hurt. But its just annoying. Any ideas how to stop it ?