In a heap of books that I imported the were separated (ineffectually as has been pointed out by a comma, These do show up in manage authors as multiple authors although as you have stated calibre thinks they are one author with a long name
. This is where I discovered the whole issue of the use of the ampersand. Manage authors has been a good source to find these problems as a thousand author entries is easier to browse than 5000 books. You just have to go back to metadata input to correct it.