How do you stop two budgies from fighting?

Border Collies are well known for being same sex aggressive so it's best not to have them together at all. Failing that, the dogs should have their own toys, sleeping areas (securely away from each other), feeding areas and so on. Constant supervision is essential and you should be able to stop any challenging behavior before it escalates. When you can't supervise, the dogs should be separated and confined to their own separate space.

Every time your dogs fight, it will reinforce their reasons for fighting and make it so much more difficult to train out, it is essential that you do not allow it to happen again.