Update. Duh!!!!
It occurred to me tonight that, since the CPL has the polarizing filter in front, if I put the PL in front of the CPL that I should get the double polarizing effect. Sure enough, it works. If I put the PL behind the CPL it doesn't work.
Hopefully I'll get a chance this weekend to go up on one of the freeway overpasses and take a time exposure to see what the freeway looks like empty. Maybe like something out of a sci-fi movie.
In the past I've used two polarizing filters to create a variable neutral density filter. With this I can darken any scene by any amount to lengthen my exposure time. It's good for taking photos of street scenes where I want to remove the people and other moving objects from the photo.
I'm not sure where I got those filters but what's in my bag now are a Circular Polarizing Filter and a Polarizing Filter (PL). These, in combination, act more like a single circular polarizing filter. In doing research I found that the polarizing filter (PL) is a simple polarizer. The circular polarizer has two parts, a polarizer and a quarter-wave plate to help the camera's autofocus feature. The quarter-wave plate defeats the dual polarizer technique.
Therefore, to create my variable neutral density filter I'm going to have to get a second PL filter.
Wiki http://en.wikipedia.org/wiki/Photographic_filter also does a good introduction to filters with special attention given to polarizers.