I have the same problem.
I have model with two layers with two different materials (lower layer -Kevlar, upper layer - Paint).
Now lower layer affects on upper layer (displace, specular). (Picture 1)
But I want lower layer shouldn't affect upper layer at all where two layers have intersections.
Upper layer must look like (Picture 2).
I got the result I need (Picture 3) with freezing/masking upper layer, but I want to know, may be for the last three years appeared a simple way to paint one layer over another without any interference?
Sorry for my bad English.
Thanks for the help.