No, Newfoundlands are not aggressive. They are very peaceful dogs that are friendly with other dogs, people, and children.
Although Newfoundlands have a gentle temperament, males sometimes try to show dominance over other male Newfoundlands. When a Newfoundland wants to show dominance, it will lunge, growl or even try to bite another male.
If you plan on having two male Newfoundlands, make sure to train them not to lunge, growl, or bite in order to prevent any fighting.