Good editors always have some support for showing multiple editor windows. This is supported in many ways. See this for a comparison of the available document interfaces.
I was happily surprised to find that Notepad++ also supports this. The trick is to right click on an edit window’s tab and choose “Move to other view” or “Clone to other view”.
By the way, Eclipse IDE also supports multiple docking of edit views. You can even open two views into the same file. However, Notepad++ allows you to synchronize this. Which, afaik, Eclipse does not.
While the above is great, sometimes you want two Notepad++ instances to, for example, open them each in their own display monitor. For that you just have to add to the notepad exe startup: -multiInst. For example on my Windows 7 instance my Notepad++ shortcut has:
“C:Program Files (x86)Notepad++notepad++.exe” -multiInst