You're gonna be bothered by how cool I am.

Well, whether or not he "gets" the HP series and its characters (which is a whole other discussion), he's a successful (and Oscar-nominated) professional screenwriter for a reason. At the very least, he has a good grasp of storytelling technique and the structural demands of a film script. Which is all one would need, I'd think, to see what would make a good place and a bad place to split a big film into two.