Sure, I could add one, but...
Are the keyboard shortcuts for each additional editor no good?
Ctrl-Cmd-1 -> Extra Editor 1
Ctrl-Cmd-2 -> Extra Editor 2
etc.
... or are you (like me) averse to using the keyboard when there is perfectly good mouse right there?

The keyboard shortcuts are only shown in the File menu from the Main Menu, not the contextual one. Maybe that's why you didn't realize they were there? Note that you can add the default editor to this list too - Yummy doesn't prohibit that.