I've finished all the quests but the main. In the main quest I've just talked all the other towns into sending troops to help out in that town (forget the name) in the north...this by closing the gates closest to them.
So far there has been no mention or evidence of Shivering Isles. Is it a completely unattached game on the other disk? I hope not...I don't want to have to start over when I go there.