So these two are really, really obscure achievements. You actually need [DEAD WORLD] to accomplish both of these, and since you have it this next tidbit of advice should be all you need to finally 100% the game.
Head to the temple (the one orange and yellow building on the island) and when you enter it, turn around and fly towards the skybox. Since you have the null spark, you should start seeing platforms fade into existence. That's about all you'll need to know, just realize that even though the skybox seems to end at some point, you need to fly through it to follow the platforms.