The book collecting term ‘dos-à-dos’ refers to a style of binding where two separate books have been bound together so they share the lower board, which serves as the back cover to both books. Dos-à-dos originates from the French for back-to-back.
Rather oddly, when a dos-à-dos book is placed on a shelf the spine of one book faces outwards and the spine of the other faces inwards towards the shelf back. This style of binding was most common in the 16th and 17th century. These early examples are, as you might expect, often very expensive, but contemporary examples are very affordable.
The definition of a dos-à-dos book has been greatly loosened over the years and it now includes two books that have been bound together in such a way that the edition has two front covers. The reader finishes the first book (which ends in the middle of the volume), closes the book and flips it 180 degrees and begins reading the second book (which will also end in the middle of the volume). Technically calling these books dos-à-dos is incorrect. They actually have a tête-bêche binding (again from French meaning head-to-toe) but dos-à-dos has become a blanket phrase for when two books have been bound together.
The Ace Doubles series produced by Ace Books in the 1950s is a prime example of head-to-toe bindings. Ace was a science fiction publisher who would place two novels into a single volume. Each work had its own cover but production costs were kept to a minimum. Many authors would use this binding style to creatively tell a story from two different points of view. Ace Doubles are very affordable.