parkdalegardener,

Yes, those filters are common and not too expensive.

That's the easy part. As you point out, taking two shots as identical as possible is the problem.

I don't believe you need two cameras. You just shoot with the lens in "non-polar" mode, then twist the filter and shoot again. I still don't see how this can be solid, even with a tripod, since there will be a second or two or three between shots, and if shooting a live subject ... well, there will be movement.

The reason I keep fantasizing on a two-aperture or two camera setup is: some automation to take both images at the exact same second. Even then, the polarized shot will take longer, since less light gets through.