Erik Kwakkel, a medieval book historian, calls this binding technique "dos-à-dos"--meaning "back to back." He writes that it is:
[...] a type almost exclusively produced in the 16th and 17th centuries. They are like Siamese twins in that they present two different entities joint at their backs: each part has one board for itself, while a third is shared between the two. Their contents show why this was done: you will often find two complementary devotional works in them, such as a prayerbook and a Psalter, or the Bible’s Old and New Testament. Reading the one text you can flip the “book” to consult the other.