“Heightblend” means nothing here, since we don’t know how you are controlling what the height is - nor what you are actually doing.
Best case scenario. Make a black/white image, load it in as a texture. Feed it as the alpha of a lerp. Use the 2 triplanar textures as A and B.
What height blend actually implies is that the 2 materials are blended together based on the Z (or B) channel of the mask that sits on top.
A being 0 the second material. B being the same material as the top material, so that its high points use that texture.
This makes it possible to pull off effects like having Moss only on high points for instance. Or filling holes with sand.
Again, all it is is a Lerp between A or B driven by a black/white texture.