I went on the Stephanie Meyer oficial site and this is what she said (direct quote):

Just a quick note on the subject of the Breaking Dawn film: there is no drama over whether the book should be one movie or two. My personal feeling is that it would be very difficult to cram the whole story into one movie (as I've said in many interviews previous to this), but if a great way of doing that surfaces, I'm all for it. Two or one, whichever way fits the story best is fine by me, and everyone I've spoken with at Summit seems to feel the same way. We're all excited to move forward on this, and we are slowly and surely getting there. I know people are anxious for news, and so sometimes gossip gets fabricated to stir things up, but there's no basis to this particular story.

Steph

so the answer is even they don't know or haven't made a final decision. Though I would add that the author and studio both seem to want 2 movies. At least thats what I took away from what I read.