Other forms: duologies
A duology is a pair of related literary or artistic works, such as a book or a movie and its sequel. A duology has a Part One and a Part Two.
The word duology is modeled after trilogy, a series of three works. It combines the prefix duo-, meaning "two," with the Greek suffix -logy, which usually pertains to writing or discourse. While the term can apply to films or plays, or even other works of art, it is most frequently used in the publishing world to describe a two-book series.