I think most dogs enjoy the company of other dogs and benefit from having another around.
Your lifestyle sounds perfect for a Newfie but can you afford the cost of a second one? Think about twice as much food, toys, bedding, vaccination fees, wormer, flea protection, spay/neuter fees, ongoing vets fees, kennel fees, grooming fees, etc.
You don't have to make the decision between one or two now, you could get the first one and then after 6 months or more, you might want to think again about getting another.
Generally, two of the same sex don't get along as well as a male/female would. Are you considering two from the same or separate litters? I have two from the same litter and I have not had any problems so far (they are almost 17 months old now). While some say they don't bond as well to you, this has certainly not been my experience.