You can split the view by clicking on Split in the tool bar or, if you have enabled menu bar, View -> Split (keyboard shortcut F3). To change back to only one folder view you can click on Close or just press F3 again. The symbol will show you which view will be closed. Depending on the currently active view the minus sign will be shown in the left or right part of the symbol. The active view will be closed. This is important to know if you want to use the keyboard shortcut. There is also a button for splitting and closing the view situated in the toolbar. This button displays the minus symbol as well, so it is easy to know which view would be closed.
