Regardless of the stone manufacturers recommendations, the effectiveness of the WRB can be compromised by the application of a cement based product. A double layer of WRB assures that the bond between the cementitious product and the water resistant barrier doesn't reduce the barriers effectiveness by providing a proper drainage plane.
http://www.benjaminobdyke.com/resources/167/HSWP1109.pdf
This is from the article. I highlighted the key points.
Felt as a product has evolved over the years. While once designated by weight – the 15-pound variety weighed 15 pounds per square (100 square feet) – modern-day equivalents are differentiated by number, i.e., #15 or #30, but weigh only 7.5 -12.5 pounds (#15) or 16-27 pounds (#30) per square. Because they weigh less, they also are less absorbent than earlier varieties, which may explain why some older felt-wrapped homes have better withstood the ravages of moisture over time.
These products do have a couple of unique features. One is they tend to improve their performance on the passage of moisture vapor (or MVTR - moisture vapor transmission rate) when they become wet. In addition, they tend to wrinkle when they are wetted. When installed in two layers, like in stucco or adhered masonry veneer, this wrinkling can create drainage gaps between the two layers. Both felt and Grade D building paper tend to be less durable than engineered, plastic housewraps (see below). Some building papers are not readily available in all areas of the country – they are widely used in Western U.S. and other areas where stucco cladding is favored, but less so in the Eastern U.S. and Canada.
2. Housewrap
Housewraps originated in the 1980s to serve as air barriers, and later evolved into moisture barrier products. Today they are becoming increasingly popular for their durability and ability to block water that has penetrated behind the exterior cladding from moving further into the exterior wall assembly. Housewraps are engineered plastic sheet membranes of varying sizes (typically .005-to-0.015 inches thick) that are wrapped around a house or other low-rise construction. They are specifically meant to resist the movement of water in the liquid state while allowing the movement of water in the vapor state.
Housewraps are designed to block water from reaching the sheathing and framing while allowing the free passage of water vapor so that wall cavities and framing lumber can dry to the outside of the building, reducing the possibility of mold and rot. Acting similar to a GORE-TEX® jacket, housewraps are water resistant on the outside, but also allow water vapor to pass through the building envelope in case moisture problems arise.
It is worth noting that housewraps are not designed to channel the direction of water movement. Since housewraps don’t allow the selective passage of water in one direction
or the other, moisture will move only in response to concentration or thermal gradients. While they resist liquid moisture movement toward the sheathing and framing, by
themselves they do not create a drainage space to purposefully channel water out of
the wall structure.
In addition, the passage of moisture vapor is two-directional. While
helpful when allowing moisture vapor from inside the structure to exit, it can also allow moisture vapor to be driven into the wall, for instance when a wet stucco wall is hit by sunlight (solar drive).
Also, as moisture moves through wood and stucco, it extracts surfactants, including detergent, oil, resin, plasticizer and colorant. Because these additives greatly reduce the surface tension of water, a housewrap can eventually lose its repellency and allow water to soak through to underlying sheathing.1 For wood siding, some manufacturers recommend priming the back of the siding with a water-repellent primer to help combat the problem.