The writers of a correspondence and the participants in a debate are to be considered as joint authors.
Ex.
Schiller, J: Christoph F: v. Briefwechsel zwischen S. und Cotta; herausg. von Vollmar.
— Briefwechsel zw. S. und Goethe. Stuttg., 1829. 6 v. S.
— Briefwechsel zw. S. und W: v. Humboldt. Stuttg., 1830. S.
Cotta. Briefwechsel. See Schiller, J: C. F: v.
Goethe, J: W. v. Briefwechsel. See Schiller, J: C. F: v.
Humboldt, K: W:, Freiherr v. Briefwechsel. See Schiller, J: C. F: v.
Many catalogues adopt the form of heading
Schiller, J: Christoph F: v., and Humboldt, K: W:, Freiherr v. Briefwechsel. Stuttg., 1830. S.