There will only be 1 ending to the game, but the player is free to a certain extent to go through the game as he wishes. There are of course certain goals every player will have to achieve, that is what having a main story is all about, but there will be lots of optional things to do and different ways of solving certain problems, just like there was in Divinity 1. Another reason for the 1 ending is because this story is part of a larger one which started in Divinity 1, and will continue in Divinity 2.