Hi Stu,
Just place your mouse cursor over the split in the feature manager tree and then click and drag either up or down the screen until one of them disappears. I don't make use of this often enough to recall offhand how to avoid this occurrence altogether but I do know that some RMB selections in the assembly mode cause it to happen automatically.
The one example I know about for certain is that RMB clicking a component (part or assembly) in the FMT and selecting "View Mates" splits the screen in order to display mate info for the selected component. If you "Edit Definition" of a mate displayed in this window the split remains but if you're just viewing the mates then you should be able to click anywhere in the graphics area and the FMT reverts to a single view.
No doubt there are other contexts that bring about this variation in the UI however as I mentioned I am now fully versed on them.
Chris Gervais
Senior Mechanical Designer
Lytron Corp.