In the anamie, I think that this fight is a lot like the fights of mew and mewtwo, kyogre and groudon, raquaza and deoxes (I think), ho-oh and lugia, and palkia and dialga. No one can win. They are evenly mached.
Like the Yin and Yang they symbolize, they balance each other out, so it's 50/50. In my White game, I was able to beat N's Reshiram with Zekrom. I'm sure I'd be able to do the same in a reversed situation.