To duplicate a window, click Window on the file menu at the top of the screen, then Duplicate Window. You can also use PC keyboard shortcut Ctrl+Alt+K. This will create a separate window of the same file, with the filename YourFileName:2 (or 3, or whatever copy it is). You can browse between the two by selecting them in the Window menu, clicking the appropriate timeline tab, or (if you have this option turned on) alt-tabbing to the separate windows in your taskbar.

