Re: Back-To-Back Sequels
You're essentially taking two ideas and confusing them. The term 'back-to-back' doesn't mean taking one story and splitting it in half. It means shooting multiple films as part of a single production period.
As to the question, I really don't see the point of back-to-back filming.
As to the idea of taking a single story and splitting it in half, I again see no point. We're not dealing with something that is too complex that it can't be covered in the course of a single film. Anyone who doesn't see how they can do Gwen's death, the Goblin stuff, Peter's growing close to MJ, etc. in a single movie (likely TASM 3) and do everything justice isn't thinking hard enough.
Also, while we're on the subject, there was absolutely no reason to artificially create two separate films out of Breaking Dawn, and, although I haven't read The Hunger Games trilogy, I don't see any reason to split Mockingjay into two parts either.