I might have to be the bummer. I'm not saying it can't work. BUT when it was time for me to get another dog, everyone advised me not to get another male, as the original male is likely to have a problem with another adult male coming into the house. I didn't listen. I took Manson with me when I went to get Bosco, and I would not have taken him if they had hated each other. Well, they got along great-- for 3 days. Until they realized that they were both living here and neither would give up the dominant position between the two. Keeping them separated -- very separated-- they cannot just be on opposite sides of a door, is not easy and with children, mishaps do happen. The mishaps are gruesome and result in large vet bills and injured dogs. I told everyone I can handle it, and I can, but in the future I will not make the decision to bring a new adult male into a home with an adult male already living there. I am all about adopting older (as in not puppies) dogs, but after some experience and learning the hard way, I would only bring in an adult of the opposite sex from the dog that already lives there.

Are mastiffs normally dog aggressive? Are danes?

here is a good article on mastiffs. the second paragraph "Do I Really Want a Mastiff? Why Do I Want a Mastiff?" goes along with what shiny says about the male -female issue. they may get along, but if they choose to fight, you have two giant breeds going toe to toe and it could get ugly. they may get along without and problems. you just cannot be sure. most mastiffs are refered to as "gentle giants" and are very affectionate dogs.

 http://mastiff.org/exhibit-hall/rescue/right.mv