If you've installed bg2-sofA and then install the expantion, you CAN go to the other areas of Amn first. But when you start with the areas for the expantion, then you CANT get back to the areas of Amn (in the same game that is).
When you start a game, you can choose to start amn or the expansion. If you start Amn, then you start from the beginning and play 7 chapters of the Original BG2 before starting the last 3 chapters of the expantion.
