I have done some experimenting with multiple circular polarized and
ND filters and their effects.

The name of the result is the same as what is formally known as a
"Fader Filter". This has the same effect but in one compact filter. The
same effect can be seen in the small window in aircraft doors, the one
with a small lever around the window. Rotate the lever and the
incoming light is reduced, almost to zero. Rotate it back and more light
passes through.

I have searched for a fader filter for many years now in ebay and
ricardo, without luck.

In order to create a fader filter today you must be a little creative:
1. Take two like polarized filters.
2. using a threaded adapter (outside threads) mount the two filters
face to face (using their inside threads).

Remember, there are two rotating surfaces now. Using them with
some practice will give you some interesting variable ND effects.

Lastly, you may have some color issues with this, especially at the
dark end of the scale. Take this into consideration and shoot RAW to
allow for color temperature adjustment when editing.

Good luck, let us know how it works for you.