Different WorldGenerators Working on the same World?

I want to know how I can make a world that is half FLAT and half DEFAULT. I have been exploring the chunk classes deeply and I know how to get the "short[][]" list of blocks but how do I put them into the world?
Where I am stuck at:

Code:java

 
WorldCreator space=new WorldCreator("Space");
space.generator("CleanroomGenerator:.");
WorldCreator flat=new WorldCreator("Flat");
flat.generator("CleanroomGenerator:20,stone,3,dirt,1,grass");
short[][] blocksSpace = space.generator().generateExtBlockSections(world, null, x, z, null);
short[][] blocksFlat =flat.generator().generateExtBlockSections(world, null, x, z, null);
 

The blocksSpace contains all the blocks in the space chunk
And blocksFlat contains all the blocks for the flat chunk
Thanks guys!