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 being one discipline, it involves two: one focusing on texts with literary scholarship methods and another concentrating on music with musicology principles. The "-ology" term suggests they might publish in similar journals, participate in the same conferences, or contribute to hymnal committees.