It is normal for budgies to bicker amongst themselves, but if your birds are actually becoming violent they should be separated immediately. Generally, two males will get along better than two females. In a male/female pair, it's common for the female to become a little bossy and pushy with the male. These are things you should keep in mind when choosing your birds.
Giving each budgie its own food dish, providing plenty of interesting toys, and having a spacious cage can all help decrease the amount of bickering amongst your budgies.