I'm not sure you can change the default double click (open) action without changing the default program to be "Notepad", but this would also effect "Merge" by changing "Merge" to "Open".

Hello SLo, and welcome to Ten Forums. :)

You might see if the "Open with Notepad" context menu added by the tutorial below may work for you for this.

Open with Notepad - Add or Remove from...