Help with Biome Features

I'm making a new Nether-type dimension. I wanna make it so that in the dimension there will be large patches of blocks that replace the main block in the dimension on occasion. I also want these patches to consist of two blocks. (Best way to describe this is I wanna make it so that on occasion, Netherrack is replaced with patches made up of sand and gravel). However I can't seem to make them even appear in the dimension, let alone make it so they're made up of 2 different blocks. I've tried putting it in Raw Generation, Surface Structures, Underground Structures, and more in terms of generation order but that doesn't seem to help either.

Can someone tell me how to make this work?