I don't know for sure, but in this video there is a part on combat. It shows just the two of them going against Five or six monsters. A summon is used in the fight. http://www.youtube.com/watch?v=weUPfejQ7xE Very curious about this also.
Enemies will not scale to your level or party size (there should be a difficulty setting, though). The game will be balanced to be challenging with 2 characters, with the expectation that most will add a henchmen/companion or two.