How do I open 2 instances of the same file in notepad++ side by side with their own scrollbars in a single Notepad++ window?

Problem

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 t…