We have two intact male newfs, one 19 months and the other 11 months old. We have never had any issues as far as dominance thus far, but we are just now starting to see some more agressive behavior. Like luvxl k 9 stated, if you assert yourself as the Alpha then there is no question as to who is in chage. However, I also tend to believe that the majority of it depends on the personality and behavior of the dogs themselves. My MIL has Alaskan Malamutes and she has issues with her 2 females. I'm definitely no expert as to behavior and such but I would imagine that it would be no different with the opposite sex.
I saw your earlier thread about questioning whether to add another pup, I thought about responding, but figured that others had already given you some great advice. I will say that it can work and does work in many homes. For us, and our 2 t's, as we call them, it does work. But it also has a lot to do with the personalities of the 2 newfs that we have. Tsunami is the erratic bully and crazy newf, and Typhoon is the most laid back newf/dog I have ever seen or been around in my life. I would say that Tsunami was almost complete with his training when we added Typhoon. He had bonded well with the whole family and still bonds daily. When we added Typhoon, it was like he found his long lost friend that he had been searching for in puppy tranining. So it was a match made in heaven so to speak. They are best of buds and love playing, sleeping, eating, and getting into mischief together. I would stress as someone else mentioned, you must spend individual time with each though.
Sorry, got off the subject a little. I guess what I'm trying to relay is not to stress to much over it. Yes, think about it, plan it to an extent. But the most important thing for you to do is make sure that you are ready for all that is involved with 2 young newfs. You definitely have the planning down with these questions you are asking. Don't let the extremes weight you down. Yes there are situations where males don't get along, but then again there are situations where a male and female don't get along or 2 females. So if you do want to add a young pup to your household, visit the pups parents, check thier personalities, check the pups personalities, ask the breeder what she or he would recommend, and then let the journey begin. JMHO on the matter. Hope this helps a little.
