I think Spiratone actually used to sell a "variable ND filter" that was just two polarizers, one stationary and one rotating, mounted to be used in this way.
Cameras that use beamsplitters for metering, like Canons from about the mid-1970s need circular polarizers.
The thing is, if you're going to carry a second filter anyway, and it's really no big deal to carry two or three filters, you might as well make it an ND and do it properly than to try to use two polarizers, because a continuously variable ND filter is rarely necessary.
If you find yourself in this situation often, then just don't shoot 800 speed film as your standard film.