I have noticed that dimension specific seeds control the terrain generation and the biome layout while the main seed parameter controls the population of chunks (caves, villages, spawners, animal placement, etc.).

When I try to import a .json file that contains an Overworld specific seed different than the main seed, the world creation process uses the main seed anyway (resulting in a different biome layout than expected). However, using an NBT editor to change the seed manually and clearing all the region files forces the game to recreate the world with the right seed, specified under Overworld section.

I'm attaching an example .json file that contains settings specific to this case.