You're probably better off using a power fractal as a blender for these two terrains, where 'the other one' is inversely blended. You may also have to play with the smallest size, it may be far too small to get a nice distinction. Try 100/1000/100 for sizes for a starter.
Another way of mixing these is to use the displacement intersection feature of a surface shader, adding the dunes as a child of the surface shader. Try it's effect with the test color first.
You could also post your tgd. I'm sure there's someone willing to have a look.