Most details are in the article ^
Reddit source of images: https://www.reddit.com/r/TheLastAirbender/comments/1hi2tte/more_confirmation_on_the_leaks_this_was_using_the/
I find this interesting! Post apocalyptic is a good way to “reset” the world, and the idea of twin Avatars has been batted around the fandom for some time.
I really like that. Maybe it could reflect their split personalities.