I should have made it clear that I was referring to Kindle-optimized author metadata.
On the Kindle, searching for an author is a full-text search. I've never had a problem finding a jointly-authored book using either "John Smith" or "Nancy Smith" as the search term. It could be that the Kindle is actually finding the text within the text, as opposed to in the metadata. My focus has been on presenting the authors in an acceptable fashion on the Kindle, and I discovered that this style worked best for me. I do understand that it's not a correct sorting value, but it's what's needed for the Kindle to show it properly.
Within calibre, I treat the Author field differently - I enter the author attribution as I expect it to ultimately be displayed on the device:
Code:
John Smith & William Jones
John Smith & Nancy Smith
That allows me to search for "Nancy Smith" in calibre, and for the author(s) to be properly rendered on the Kindle.
I realize that this may not be an optimal solution for folks with multiple devices utilizing different formats. Once I receive my iPad I may have to revisit this ...
G