Can two polarising filters be used as a substitute to an ND (Neutral Density Filter)?
No because offset polarizing filters do not filter all wavelengths of light to the same degree. The most visible differences are in the shortest wavelengths, (blue towards violet), which tends to be effected much less than the shorter, (red), wavelengths of light. You can see evidence of this if you set the polarizer’s at a 90 degree angle to each other and look at something like a bright light. You will see that the blue’s come though much more strongly than other colors. Neutral density filters are well engineered to reduce all colors to the same degree and if true colors are important to you that is the filter you want to use when you want to extend you exposure times. Even if you are shooting Black and White there is a difference. Blue may not be a color in a Black and White picture, but it is a shade of grey. If it is shown as brighter than it actually should be, (which is what offset polarizing filters will do), than things that are that color will appear whiter than they should,
