Final answer:
The two storylines in 'Slaughterhouse-Five' involve historical fiction, with World War II themes, and science fiction, involving abduction by aliens, reflecting English literature's New Historicism that explores the interplay between history and literature.
Step-by-step explanation:
The two storylines that occur simultaneously in Kurt Vonnegut's Slaughterhouse-Five fall under the subject of English literature, and reinforce the concepts of New Historicism which posits a dynamic relationship between history and literature. One storyline is based on protagonist Billy Pilgrim's experiences during World War II, including the firebombing of Dresden, illustrating historical fiction elements.
The other storyline is science fiction in nature, involving Billy's abduction by aliens from the planet Tralfamadore and reflects concerns of the time in which the novel was composed, such as the sense of fatalism and questioning of free will during the postmodern era. This narrative structure allows Vonnegut to conduct a parallel reading of history and reality, exploring how literature not only mirrors historical events but also shapes society's perception and attitudes toward them.