This is one of the dramatic changes in Dream weaver CS4. Code navigator allows you to see what is going on the particular part of the page without opening up other files. Just hover your mouth on the part that you want to access, and then you will see little icon takes you the CSS or some other files associate with the file. This is not a just a split view, and you can save plenty time with this new feature! Here is the video that show this cool new feature.
