To be honest, it does not take a año to produce a film and editar it. This is Summit's plan of trying to leche the Twilight Saga of what it's worth. However, this isn't the best plan, considering, the momentum of the series is slowing down. I mean, look at the Twilight series spot now. All the fansites. Hardly any news, therefore, fans become uninterested until things pick up again.
So in answer to your question, there isn't rally a good reason why they are releasing part 2 a año later. Splitting the movie adaption into two cine might serve the story justice (we'll see), but releasing them a año apart is unwise.
The book was too long to make into one movie...they would have had to cut most of the story out....two movies...more of the story and más money for Summitt ....lol
Its there way of making más money, they are taking advantage of the movies because they are soo popular summit know people are going to pay to see it :)
I think they decided on doing them a año apart instead of 6 months because there are supposed to be a bunch of blockbusters siguiente summer. I'm not sure exactly what, but that was what the official release said.