1. Right-click any Notepad++ shortcut.
2. Select Properties.
3. Move to the Shortcut tab.
4. In the end of the Target field, add ” -multiInst”.
5. Click OK.

Example:

• Target: “C:\Program Files\Notepad++\notepad++.exe” -multiInst

以多个实例方式打开Notepad++ 
http://blog.csdn.net/wdjhzw/article/details/30314657

 when already using the editor, you can simply press ALT + F6, or Click Run -> Open in another instance in the menu bar.

or :

以多个实例方式打开Notepad++