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.
Sounds a lot like an idea I had after finishing Korra. With both Raava and Vaatu in the Avatar line of reincarnation now, I had once thought it would be interesting if, at some point down the line, there could be twin Avatars who each contained one of them. Maybe each only capable of using two elements rather than all four.
That sounds amazing and perfectly in line with the canon!
I really like that. Maybe it could reflect their split personalities.