Hmm. That's interesting, but it goes against the concept of 7 being the magical number, plus all the movies are by year, too, so would it be year 7 and 7.5?
I can't see them doing it though, because there was so much left out of OotP that should have been there and stuff left in that wasn't too important and that realistically could have been a 2-parter. But we'll see. We've a few years to go before it hits the screens.