It took a while to find the exact setup for the new plugboard system to handle multiple authors on the Kindle so I thought I would post it here for anyone else wanting it.
1. Set the MOBI output 'use author sort for author' flag to checked.
2. Create a plugboard "mobi, any device' (since there isn't a kindle 3 option here, i needed any device)
3. Enter this text in the right side field: {author_sort:re(\s\&\s,

}
(after the comma is a semi-colon and a right-paren, the webpage is showing it as a smilie and I don't see how to disable that formatting)
4. Set destination field combobox to 'author_sort'
5. Push the file to the Kindle
6. You may have to reboot the kindle to see the changes. (home, menu, settings, menu, restart)
This will convert "LN1, FN1 & LN2, FN2" to "LN1, FN1;LN2, FN2" which the Kindle will display as "FN1 LN1 and FN2 LN2"