Mark Morris’ identity as an artist is inseparable from the music. Not only does he make dances, he conducts their scores, and “Mark Morris: Two Operas, An evening of Britten and Purcell” was a personification of this artistic Janus. Each work profited from his devotion to both opera and dance, but in opposing ways. Morris highlighted the music in his New York premiere of Benjamin Britten’s “Curlew River” and focused on the dance in his 1989 classic, Henry Purcell’s “Dido and Aeneas.” In both cases, though, the partner art was rendered with beautiful attention.