The bookbindings above are as odd as they are rare. In fact, I encountered my first only a few days ago while browsing Folger Library’s image database of bookbindings. The binding is called “dos-à-dos” (back to back), 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...Photo and text from the ever-interesting Erik Kwakkel, where there are additional photos of similar bindings, including one that incorporates seven texts back-to-back-to-back-to-back-to-back-to-back-to-back.
Addendum: A hat tip to reader Christina Wilsdon for her observation that the term "dos-à-dos" (meaning "back to back") is related in etymology to the square dancing term "do-si-do."