The literary term diptych has been credited as being coined by A.C. Spearing in his 1981 critique, The Awntyrs off Arthure. In this analysis, Spearing says, “We seem to be faced with a literary structure comparable with a favorite pictorial form of the Middle Ages, the diptych.” Strangely, he makes this comparison after spending two pages providing evidence that the poem, Awntyrs off Arthure was written by a single author and not multiple authors as was previously believed. It is as if this was more of an afterthought than a main concept.
Although Spearing is credited with identifying this concept, he may have had some help in this matter. Years earlier in 1971, Alan Soons published an article in Orbis Litterarum in which he argues (briefly) that some Biblical texts were intentionally written to have a layout “similar to that of a diptych.” It is possible that this article influenced Spearing in some way.
Over time, more and more people have applied the term diptych to various forms of literary work, especially medieval poetry. It is no longer one man's concept. It is now considered a valid ideal in medieval literature.