They may vignette a little on wider angle lenses due to the thickness of the edge rings. Yes two polarizers are the trick for getting light reduction and having exact control of how much reduction you want. Be aware that two polarizers will take off about 3 1/2 to 4 f stops of light, even turned to their brightest.
Be aware that two polarizers will take off about 3 1/2 to 4 f stops of light, even turned to their brightest.
I don't think they cut off quite that much light ...
I recently experimented with two stacked polarizers (one circular, one linear); I don't know the absolute exposure reduction, but between the most "open" and the most opaque settings of the two filters was a difference of about 4 stops.
Be aware that using polarizers in this fashion can add a significant blue color cast, depending on the angle of the light source and the filters.
ISO 80, f/8.0, 2 sec (adjusted): ISO 80, f/8.0, 4 sec, unadjusted:
