To display two different scripts side by side, open one of them and then the other. At this point you may only see one of the two.
In Final Draft 13 / 12, go to View > Arrange Open Documents and choose Tile or Cascade. All open scripts will be displayed in their own resizable and movable document window. From there, the windows can be resized and moved to the user's liking.
In Final Draft 11 and older, click the Restore Down icon at the upper right to render all open scripts in their own windows, which can then be resized and arranged