The Save Copy As command in the File menu works just like File|Save As. The only difference is that after File|Save Copy As, the File|Save command will continue to save the file using the original file name. The file name indicated by the tab does not change.
The Save Copy As command adds an extra "open the saved file" checkbox at the bottom of the file selection screen. If you turn on this option, EditPad opens the copy of the file that you just saved. You'll end up with two tabs, one for the original file, and one for the copy. You can edit them independently.
See File|Save As for more information on how EditPad saves files. See View|File History for a quicker way to save milestone copies of your files.