To be honest, it does not take a năm to produce a film and chỉnh sửa it. This is Summit's plan of trying to sữa 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, những người hâm mộ 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 năm later. Splitting the movie adaption into two phim chiếu rạp might serve the story justice (we'll see), but releasing them a năm apart is unwise.
I think they decided on doing them a năm apart instead of 6 months because there are supposed to be a bunch of blockbusters tiếp theo summer. I'm not sure exactly what, but that was what the official release said.