I think it totaly depends on the dogs, if you can get a male/male. I had two males and they got along great. One was the alpha and the other was fine with it. Cisco is the alpha of the house now, and if I brought home another male that wanted to be in charge, there would be problems for sure. Cisco is very dominant over my 3 yr old female pom, and she just backs down from him. (she is large for a pom, about 25 lbs) He is just now 10 months and I have very recently noticed his "man in charge" attitude starting to show.
I have heard what you should never do is have two females, even with cats.