[Les Meehan]

This is definitely two polarisers working together to control the light transmission. If you look at the top left corner rocks in the last two sample images you will see a noticeable polarising effect on the wet rocks. In fact, since the surface angle of the rocks varies so does the pola effect (which is normal) but this only goes to highlight that they are polarisers.

I have often used my pola filter as an ND when I needed to change exposure settings for whatever reason. It may be possible to obtain a variable ND using two ordinary polas, and it would be cheaper!

Most people think of polarisers as colour saturation improvers, which they are, but in fact what they really do is control reflections. A side effect of this is increased colour saturation. It is useful to think of them in terms of their ability to control subtle highlights on things like foliage, painted metal etc. It isn't always the best creative option to use the maximum polarisation of a filter as it can kill the subtle reflections from the myriad surfaces in a scene and make it look 'lifeless' and unnatural. Just an aside about polas, sorry!