The only way we'll know is if there's a third legendary.
The 3rd game is always centered on the third legendary, ever since 3rd gen. So far, there have been 3rd games in 3th and 4th gen. In 5th there were sequels because of the nature of Kyurem. Unlike past legendaries, Kyurem was just a husk waiting to be filled with the power of another legendary and, because you could fuse either Reshiram or Zekrom with it, it made much more sense for GF to make a pair of sequels than a third game. However, despite being sequels, B2W2 still retain a good deal of similarities to BW. Quite a bit of dialogue is the same, and the path you take through the game is much similar to the first games until you beat Skyla.
Deep within the game there will be a 3rd main legendary that will be the key to knowing whether we'll have a 3rd game.