Say A is the base, C is the overlay, and B is the edge around C.
- In a composite map plug in the A map.
- Make a noise map.
- Pipe it through an Output map and adjust the curve with two extra keys so it goes horizontal-steep up-horizontal. Use this as the mask for the edge (B) map and plug that in the Composite above A.
- Copy that Output map using the same noise input and move the two keys slightly to the right, now use this as a mask for the overlay map (C).
The Composite map will be stacked A<-B<-C with the two Output maps with the noise for the B & C masks.
For materials you can use the Blend Mtl in the same way using those same masks.