The sense of writing partner here is similar to that mentioned by Niklas Luhmann in Communicating with Slip Boxes: An Empirical Account (1981), though in his case his writing partner was a carefully constructed database archive of his past notes.
see:
Luhmann, Niklas. “Kommunikation mit Zettelkästen: Ein Erfahrungsbericht.” In Öffentliche Meinung und sozialer Wandel / Public Opinion and Social Change, edited by Horst Baier, Hans Mathias Kepplinger, and Kurt Reumann, 222–28. Wiesbaden: VS Verlag für Sozialwissenschaften, 1981. https://doi.org/10.1007/978-3-322-87749-9_19.<br />
translation at https://web.archive.org/web/20150825031821/http://scriptogr.am/kuehnm.