I said,
will save two copies of the file whenever a save is triggered – it will write the main file, and it will write to the defined backup location:
So, that’s technically true, but I had forgotten a small caveat, that came as I was verifying my assertions in a portable Notepad++ with AutoSave turned on to save every 1 minute:
What happens is that when a save is triggered (either from the save button, File > Save, or the AutoSave plugin), Notepad++ copies the current contents of the file on disk to the backup location, then saves the new edits to the original location – so the backup is always one version out of date w/r/t the main file. If that’s really what you want, then my description still holds – it solves 2/3 of the problem.
If that’s not right, and you really want two identical copies of the file saved, it would have to be a custom solution, because saving two identical copies in two locations, every time a file is saved is not a normal feature of a text editor. However, that could, theoretically be implemented by making a custom plugin, or (easier) through hooking into either the FILEBEFORESAVE or FILESAVED event using one of the N++ scripting plugins, like PythonScript or LuaScript.