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Years ago I used dual polarizers as an infinite ND filter but it doesn't work with CPLs.
It won't if you mount both in the conventional orientation. You can either use a linear polarizer for the front filter with a circular pol behind it or invert the front circular polarizer.
One thing left out of the descriptions of polarizers is the circular polarizing element. All of the descriptions apply directly to linear polarizers only. Circular polarizers have an extra element in the sandwich.
Polarizing elements, the "grid" referred to in the B+W quote, are extremely fragile. Simply touching them will destroy them. They must be laminated with something durable to protect them. There are two methods used for photographic polarizers. One is to laminate the polarizing film between two uncoated sheets of glass and then polish to two outside surfaces flat and parallel. This leaves an uncoated filter, but it results in little optical problems otherwise. The other method is to coat, or mult-coat, one surface of each glass wafer before laminating the film between them. This results in a coated filter but keeping the two outside surfaces perfectly plano-parallel is rather difficult. Any error destroys image quality. The nature of both how coatings are applied and the polarizing film generally makes coating after laminating impossible.
Circular polarizers differ in that they have a second element behind the polarizing element for a total of 4 components. This is the "circular polarizer". It acts to de-polarize the light passing through it. The result of the completed sandwich is that the first element, the so-called linear polarizer, selects the desired portion of the light and the second element, the circular polarizer, scrambles the orientation of the light so that it doesn't create problems for the AF, metering, and viewing sysystems in the camera.
The lamination necessary is a large part of why polarizers are more expensive than simple filters made from dyed glass. Additionally, the circular polarizing element is rather difficult to make if you want it to be perfectly neutral in color. You will find that, in general, the more expensive filters are the mos neutral. Inexpensive circular polarizers very often have a distinct color tint, often yellow/amber. A third issues is sealing the outer rim of the sandwich. The cheaper filters are generally unsealed and have raw glass edges which reflect light internally (this is also true of inexpensive simple filters). The better filters have some form of edge sealing. B+W has a special sealing technique used in the Kasseman versions which is supposed to be better than other methods.
The quality of any filter, but especially a polarizer, is as important as, or possibly more important than, the quality of the lens used. All filters always degrade the image quality, period. The differences are only in the degree of the loss.
