Stefan, a single polarising filter is not the same thing as working with two. A single filter is the same a using a neutral density filter, and that's it. A pair of polarising filters are used in tandem, with one rotated with respects to the other - then you get the variable transmission with the change in polarised angle between the two, all the way to nearly no transmission at all.
If you were already aware of this, my apology for repeating things,
Alex.