OK, I've misunderstood you, sorry. The niche won't help or won't represent the two layers at least. As Tim suggested above, a high-priority material using a morph (so no connection issues)for each skin - 1 waterproofing, 1 for protective layer would ...

 I was thinking the niche object could help - in the image- 01 - Select the Library object called Wall Niche Rectangular02 - set width and height03 - set surface to show waterproofing04 - set depth in wall

try using the niche tool - under Windows - it's placed into walls like an opening and depth, height and width can be set along with surface colour

it's not possible to trim skins without trimming the wall. Trimming the wall can be done with SEOs but this would be time consuming and slow down your model if used extensively.

This is vital! #19year-old-tool-needs-revising!