There are two types of polarising filters. The simple linear and the compound sandwich which has a 1/4 wave filter on the camera side of the filter so the electronics of the camera will still work. The latter is often called circular but it's isn't really, there would be no point is turning a circular filter it would be either clockwise or anticlockwise but that's not the case you can turn them to filter out the light you want.

Putting two polarising filters together and turning them will work as a neutral density filter but only if there is not a 1/4 wave filter between them.