Sorry damn work keeps getting in the way! ha.
Doing some tests and kinda got what I wanted working, tried the IF node, didnt get that working, possibly inputting things wrong? Not sure.
So the issue I am having with the first image is that although the sand is fully covering up the stone tiles when it is blending and its not 100% stone or 100% sand it is actually moving the geo. The stones get raised up as they are blending into the sand. When they should remain the same height forever and only raise up when they become full sand.
Also in areas where the stones height map is very dark, like the crevasses and it blends to the sand height map, which is a lot brighter, you get spikes in those recesses and they dont blend very well. The spiking issue is the worst of it, if I have to live with the fact that the stones move up or down as the sand is blending on or away I can live with that.
Bottom part of the image is the IF node. Its not filling in the sand into the gaps.