Assuming all the author_sort names are in the format "LN, FN", then the following seems to work:
Code:
{author_sort:'strcat(uppercase(list_item($,0,',')),', ',list_item($,1,','))'}
The problem with that is if you have a book by Madonna or Cher.
And thinking about this, the following will work for those cases as well:
Code:
{author_sort:'contains($,",",strcat(uppercase(list_item($,0,",")),", ",list_item($,1,",")),uppercase($))'}