The fatter and lazier of Arcadio's twin boys, Aureliano Segundo spends his life shuttling between his wife, Fernanda del Carpio, and his mistress, Petra Cotes.
We can explore a couple of different things through this character. First, it's pretty important to deal with the whole twins-switched-in-childhood thing (which, you know, doesn't happen every day). The point isn't really whether Aureliano Segundo and José Arcadio Segundo actually forgot who was who. More interesting is the whole idea of a dual existence, which seems to be kind of a theme for Aureliano Segundo for the rest of his life.
It's not just that his wife and mistress know about each other and seem okay with sharing him. Think about how different his life is with each of them. With Fernanda, he is forced to stick to the straight and narrow and to rein in every impulse and desire. With Petra Cotes it's nothing but fun and games: drinking, sex, and partying. With Fernanda comes the respectability of family, and often the weight loss that goes with a meager diet. With Petra comes physical pleasure, a total lack of responsibility, and the massive weight gain of an unchecked appetite. Is one better than the other? Are they both equally problematic?
And then there are the super-breeding rabbits and cows. That's the other thing to think about with this character: the way the magical and supernatural elements in the novel are seamlessly integrated with characters' personalities. Aureliano Segundo is predominantly a hedonist –someone who spends his life pursuing physical pleasure. Think about all those eating contests, the wallpaper made out of money, and the nonstop sex, and it makes perfect sense that the kind of magical event that happens in his life is the out-of-control breeding of cattle. It's a way for the supernatural to explore the wild, animalistic urges that fill Aureliano Segundo's life.
But what really makes the magic/realism crossover complete is the way one element is influenced by the other. Sure, you've got cattle having babies outside the laws of nature. But in the end, they're just cattle; there's nothing magical about them. They're hard to feed, they bring their owner a lot of money, and eventually they all drown in the Macondo floods.