I have found the split screen useful in some cases
1) When you split the feature manager, each section can display different information (ie, features, mates, configs, render, cosmos, or any add-ons).
So, for example, the top section can display all of the part features that you are working with, while the bottom section displays mates. Gives you the ability to view multiple information about the model at the same time.
2) You can view different sections of the complete feature manager at the same time. For example,(if you have a large assy or so), the top section can display a part at the top of the list, while the bottom section displays a part from the bottom of the full list. This can help you mate items, (ie. if you wanted to now select the 'Top' plane from each part to mate)
You can always get rid of the split screen by grabbing the line between the 2 screens and dragging it all of the way to the top or bottom.
-Lou