Wait a minute... this wouldn't happen by any chance to be two books each with one author, but different authors for different books -- even though they are part of a series -- would it?
e.g. Forgotten Realms: War of the Spider Queen series:
Book 1 by Richard Lee Byers
Book 2 by Thomas M. Reid
Book 3 by Richard Baker
Book 4 by Lisa Smedman
Book 5 by Philip Athans
Book 6 by Paul S. Kemp
If so I think I know why we were having so much trouble.
Templates are evaluated strictly within a one-book context -- they only know the metadata for that book and cannot access the database to find out about other books.
You can only find out authors for
that specific book.
In the past, people have gotten around that by using calibre-debug scripts to open the database, perform searches, and update custom columns for matching books when certain conditions are met. For example, formatting the series number with only enough leading zeros to match the highest numbered book in a series.