I remember doing this a long time ago and have forgotten how I had done it. I like to do this when I have long html or php files to edit and I need part of the code from further down the file in a place nearer to the top, or when I want to compare different parts of the same file.

There was a way to do this without opening two instances of Notepad++ and when I clicked on save, it made the saved changes in both instances of the opened file (whereas if I have 2 windows of Notepad++ opened simultaneously, it will prompt me to either update or not update the second opened instance if the first one was saved midway.)