In my twenties I listened obsessively to Jussi Bjoerling. Listening to him now I see why. The elegance of his glissandi is wonderful. And he puts that catch in his voice in just the right places. His touch is completely flawless. And certainly pairing him with Victoria de los Angeles is pure genius.
Each singer has his or her own career, but only certain pairings are greater than the sum of their parts. These decisions usually depend on recording contracts and not on art.