[Les Meehan]

I don't think this would work with circular polas. The idea is that the lines in the pola interfere with each other which I don't see happening with CP's. But then I'm not an expert on CP's so this may be wrong but it makes logical sense to me.

In my professional opinion, I would go for high quality ND's in different strengths depending on your requirements. Also, remember that adding ND's together simply increases the effect i.e. two 0.3 NDs makes one 0.6 BUT of course the optical quality might reduce slightly because of the extra surfaces (but probably not enough to bother you). If adding two filters together any gap between them will more than likely incur internal reflections. I once saw a good example of this whilst using two Kodak CC gelatins with a shot including candles. Had multi images of the candle flames.