I think it really depends on the individual dogs. In most cases, males will co-exist happily enough. It's not to say that fights can't happen, but generally with this combination, once they establish what the hierarchy is going to be, then they put the argument behind them. Bitches, on the other hand, do not put it behind them - and that's why it's such an issue (ie. it isn't just that problems occur more often, but that they won't stop fighting if they start).
Anyway, the most successful combination is usually opposite genders. Two males is next. And two females is a distant last.