Do you mean you get two previews, one with the source and the other with the timeline?
If so, there is a "Dual View" icon at the top right of each preview window. Click on it, or press CTRL-D. The keyboard shortcut might well be how you are enabling it by accident.
If you mean the single preview switches to source view. that happens automatically when you select something in the library. Just use the tabs above the preview window, click on the timeline or drag the source down to the timeline to get it to show the timeline view.