tweequel
a story, especially a movie, which is set chronologically either BETWEEN two other stories in a series, or at some other in-between moment of another story. Similar to prequel and threequel, it is a combination of the words 'between' and 'sequel'
For example, the video game 'Star Wars: Empire at War' is set between the storyline of the two movie trilogies, and can be considered a tweequel.