Thanks for your response redwing. I also treat classical music differently than other music. Still, the concept of an "album" is important especially if it's not a compilation of performances. In my physical music library I sort my CDs/LPs by composers when the album's works are mainly by a single composer (e.g. an album of Beethoven's String Quartets or another of Bach's organ works) or by performers when there is a single main performer (e.g. a soloist, an ensemble, a conductor, e.t.c.) and works by different composers. So I find the "AlbumComposer" tag quite helpful. Nevertheless, this is my view...
But returning to the original question: how could I sort one custom field by another custom field? In this case, it is how to sort "AlbumComposers" field by its corresponding "AlbumComposersSort" field.