Rather, it's two disciplines: one that studies the texts and follows the rules of literary scholarship, the other that is trained in music and follows the rules of musicology. The "-ology" just means that they might publish in the same journals, occasionally attend the same conferences, or be asked to serve on a hymnal committee.
Rather than a single field, there are two areas of study: one dedicated to textual analysis and literary scholarship rules, and another focused on musical expertise and musicology standards. The "-ology" implies they may collaborate by publishing in common journals, attending the same conferences, or serving on hymnal committees.