Hard to envision how a database would interpret or truncate the contents differently, from one field for different usages.
How many books with subtitles would this apply to?
Copying the titles to another field (using the change/move/copy tool, and quickly editing them by removing the subtitle from the “title” field, then use either one or the other for the two purposes in the output style, is another way to do it. You might consider using the “short title” as the title alone? Subsequent use of that record in footnotes might already be using that field.