Complex sentence: The scripts for the "Heroes Join Forces" crossover were written in September 2015, with Berlanti creating the story for the event, with Kreisberg and Guggenheim helping to craft the story of "The Flash" and "Arrow" episodes, respectively.
Split into simpler sentences: The scripts for the "Heroes Join Forces" crossover were written in September 2015. Berlanti created the story for the event, and Kreisberg and Guggenheim helped to craft the story of "The Flash" and "Arrow" episodes, respectively.