Here dilogy is defined as "a series of two related works". I can't shake off the feeling that there is a more commonly used word for this. Is there? If yes, what is it?
A duology is a pair of related novels, plays or films.
"diptych", sense 3 : a work made up of two matching parts (ref: Merriam-Webster).
Two works do not a series make. In fact, two of anything do not a series make.
Carlo, however, has the correct answer here: it’s a diptych, which is like a triptych but with two pieces instead of three. Wikipedia has a brief article on it.
Here is more about its etymology, from the OED:
Latin diptycha (pl.), a. late Greek δίπτυχα pair of writing-tablets, neut. pl. of δίπτυχος double-folded, f. δι-, δίς twice + πτυχή fold. Cf. mod. French diptyque, C. 1700 in Hatz.-Darm.
Diptych does enjoy some currency as a work comprising two halves. For example:
The 2nd, and darker portion, of this volume devotes most of its space to lamentations for the poets estranged daughter, for his deceased parents, and for a enigmatic hell-raiser of a perplexing but beloved cat, a very human quadruped. — Diptych: Voyage to Ernie Spirit, Swell a Large Lament and Other Fictions
Furthermore, award-winning author Dan Simmons often refers to his two-volume stories as diptychs, such as the Ilium and Olympos pair. For example, from Dan’s own website:
Page 284 This long quote from Marcel Proust–which figures twice in the text, here and on page 459–comes from LE CÔTÉ DE GUERMANTES. This might be the crux of the ILIUM/OLYMPOS diptych, if not of Dan’s entire œuvre.
And it’s not just Simmons. Here is one critic writing about another pair of novels of Dan’s:
[A] Simmons fan will note that a similar problem affected the Endymion diptych: The first volume wasn’t terribly useful, but the second one was the whole point of the setup. — [citation]