Create a custom column, built from other columns.
Name it something like 'mainauthor'.
Put this in the template field:
Code:
{author_sort:list_item( 0, ' & ')}
Sort on your custom column at startup.
Don't forget to add a leading # to the custom column name when you refer to it.
Code:
sort_columns_at_startup = (('#mainauthor', 0), ('series', 0), ('title', 0))
Caveat: I can't check my syntax at the moment, it may not be 100% correct, but you should get the idea ...