ironsoul, that LightColab page you linked talks about the issue you just raised ... >>>The problem I found is the cross polarized image was much darker than expected (loss in the capture process ?) <<

"Another issue to be aware of is that the proportional amounts of the diffuse and spec will be off slightly. This is because the parallel polarized image lets 100% of the spec through, but blocks 50% of the diffuse, meaning that the spec will be 2x brighter, in relation to the diffuse, than it would in a photo without any polarizing filters. You can observe in the images above that the skin looks oilier than it normally would. Once aware of this, it is easy to adjust for it in your comp by either brightening the diffuse or darkening the spec a bit. In the animated image below, you can see the effects of brightening the cross polarized "diffuse" pass and the resulting composited image."