I'm kinda curious as to why they split up JL with a two year gap. Usually when they do split up films, it's a year apart. Seems like it's better if they took out "part 1" and "part 2" and substitute a real subtitle instead.
It's my suspicion that each film will further the storyline along....think of it like the Justice League movie are the main story....like Flashpoint and The Flash, WW and Batman are Flashpoint: Batman or Flashpoint: Wonder Woman.
Looking at the slate there are three films in between JL One and Two...
Batman v Superman: Dawn of Justice, directed by Zack Snyder (2016)
Suicide Squad, directed by David Ayer (2016)
Wonder Woman, starring Gal Gadot (2017)
Justice League Part One, directed by Zack Snyder, with Ben Affleck, Henry Cavill and Amy Adams reprising their roles (2017)
The Flash, starring Ezra Miller (2018)
Aquaman, starring Jason Momoa (2018)
Shazam (2019)
Justice League Part Two, directed by Zack Snyder (2019)
Cyborg, starring Ray Fisher (2020)
Green Lantern (2020)
so those films may deal with character origins but also move along the plot from JL One to JL Two...while Cyborg and GL will deal with their character origins and the aftermath of JL Two...