Except for a very short period, I've never lived with more than one dog at a time, so I don't know much about it.
But I've read (sorry, can't remember the dog book) that canine packs tend to have both a dominant male and a dominant female. That's why two dogs of the same sex tend to argue more, because they're both trying to get the top spot. The book reccommended that it's best to have a male/female combo, so neither feels overly threatened by the other. My cousin has two females, both with "take charge" attitudes, and there were a lot of squabbles between them until they sorted out their pecking order. They get along fine now, but there is still the occassional squirmish between them.