Whether we view Luke and Acts as one document or two affects how we understand their genre.
So for example, Cadbury argues against the biography genre for Luke because that genre doesn't also fit acts: “That Luke’s gospel should not be counted a formal biography is further confirmed when one recalls that it is merely part of a longer work, and that its sequel, though full of biographical incident, is even less concerned with sketching the full career of its principal characters” (Cadbury 132).
On the other hand, Aune (77) argues for history as the genre of both Luke and Acts, because he concludes that Acts is history and that Luke must go with it: "Luke does not belong to a type of ancient biography for it belongs with Acts, and Acts cannot be forced into a biographical mold."
Transitions between related works function according to the audience's expectations. To use a popular example, Star Wars and The Lord of the Rings are two multi-part works (LOTR was three separately-published books before it was three movies) that make their transitions in different ways. LOTR has one clear story arc that spans the three books, and while Tolkien uses the different books to help structure the narrative, none of them works as a narrative of its own. In fact, when the first movie came out there was discussion about how some people were upset coming out of the movie because it wasn't a whole movie on its own. The first Star Wars (1977), on the other hand, clearly is a self-contained movie. If Lucas hadn't gone on to make the others, the only noticeable differences probably would have been the curious title "Episode IV" at the outset and Darth Vader flying away alone at the end. What's the difference? Well, I think that in LOTR we expect a tighter plot among the various volumes than we do in Star Wars. The former we expect to have been written all of a piece, whereas the latter we might expect the first movie to have come first, followed by the later movies that tried to fit with it as best they could.