Hi 404, I think the only solution to this problem is to create a new "reference type" (Endnote has 3 unused ones). Select which fields you would use and what you want to call them (normally just the field name). So for example, Author would be the acronym (DCMS), then year would be year, use secondary author for the full name and the rest would be normal.
To do this, you need to go to the EDIT menu, preferences, Reference Types (down the left)>click on modify reference types>at the top for reference type, scroll down and select unused1 and off you go.
Once you have done this and saved, go to the edit menu again >output styles>edit Harvard
Under bibliography select templates, select reference types at the stop and select the new one you just made. You can copy and paste the one you have been using but after author, put the field secondary author (or whatever you called it).
Hope this makes sense??