It's called littermate syndrome, but really, it can happen with any dogs that are close in age, related or not. If you do get two puppies at the same time, it's not twice the work, it's four times the work, because you have to do pretty much everything separately.
Newfies are very companionable dogs, and were historically used more for drafting and rescue work than guarding. I know a dog of a closely related breed (Leonberger), and I can't imagine him being happy working out and away from his people most of the time. He loves the outside, but he's a big mama's boy and needs social interaction with his family.
What do you need guarding? Livestock, or just the property? What does it need guarding from (animals, people, etc.)? What do you expect the dog to do (deter/alert, chase, actively engage intruders)? The right fit will depend on exactly the kind of work you're looking for. Livestock guardian breeds, for example, are excellent at, well, guarding livestock, but may not work well for protecting territory alone. And some breeds are a much bigger liability than others - check your local laws, but in some areas you can land in legal trouble if your dog bites a person, even if that person was trespassing or otherwise breaking the law.
And any guard dog will need some training, even if they're from working lines with successfully working parents. Just be sure you're ready for that, because it's a more specialized skill set than basic manners training.
Newfies are NOT guardian dogs at all. They are very attached to their people and while they are very large, the breed is just not bred as a livestock guardian. Are you maybe thinking of Great Pyrenees? I don't think I've ever even heard of using a Newf for those sorts of things - though they can be useful on a homestead for things like hauling wood and other task like that - but not until they're at least over two years old and have been trained and conditioned for drafting.