I have a male puppy, and my spouse's parents have a female puppy from the same litter. Whenever they get together, they rough-house, and it sometimes looks a lot worse than it is. Neither one of them have drew blood, or caused an injury (other than a minor scratch) to the other. As long as these dogs are playing around like this, I feel it's ok. Animals in the wild play together all the time.
Now, if one dog comes out all bloodied with a broken leg or something, then it's definitely something to be concerned over. Or if your neighbour is purposely pitting the dogs against one another like a dog fight. If your neighbour punishes the dog(s) who doesn't win - by starving them, locking them in a small kennel, etc - then I would definitely contact the Humane Society, or animal services, or whatever is available in your area.
But like I said - if it's just rough-housing, they're just playing around. It's when injuries and punshiments arise from it that it should be a concern.