AZIndex multiple custom field values

I am building a bibliography site. I use a custom field “first author” (single value) and “author” (multiple values) for all other authors.
I would like to create an index showing all first authors and authors combined.
AZIndex shows either just the elements for the one-value “first author” or the elements for the first value of the multi-value “author”.
Is there a way to achieve the index for all authors combined?
Additional problem is, that “first author” in one post might be “author” in another, so they have to be checked.