If you put the linear in front of the circular (need the quarter wave plate "scrambler" which constitutes the back of a "circular" polarizer to be last thing in the chain) one should be able to selectively block from 50%* to 100%* of the light. When the phase of the polarizers is the same, it should be identical to using one**, you're effectively blocking all the light "vibrating" in one axis. Turn the front polarizer 90% in orientation to the rear one and you should be blocking all.
By selecting the offset between the two polarizers one should be able to pick their desired amount of darkening, and by turning the two together (maintaining their relative offset) one should be able to pick which of the remaining angles of light get passed, allowing one to block glare (and other stereotypical polarizer effects) or not.
I need to just get off my ass and order a cheap linear to see just how well this theory translates into practice.
*A perfect polarizer would let 0% of light oriented with it pass. There is no perfect polarizer, they all pass some in-orientation light and block some out-of-orientation light. IIUC polarizers sold for photographic purposes have a (in theory) 90 degree band pass. Since a sine offset by 180 degrees is effectively indistinguishable from the original (camera sensors and eyeballs are effectively blind to phase), we're only concerned with 180 degrees of coverage. Think of it as light bouncing up and down vs light bouncing side to side. (not truly true, but true enough)
**identical to using one minus the loss inherit in the films.
EDIT:
My use of the word "phase" is sloppy, but I think the point gets across?
