There is also the possibility that this whole "Two Movies at the Same Time" thing is a marketing ploy. It may be that both are really one single movie containing both Zekrom and Reshiram. I mean they never really revealed reliable information on any of the movies to begin with in my opinion. For example: the Zoroark movie was originally Pokemon Z or Trainer Z or something like that. And they used clips that weren't even in the movie at all. So that is one possibility.
And to go along with that, there are so many legendaries this Gen that they have plenty of different options for more movies within the Black and White series.