When I shoot landscapes/water, I often stack 2 or 3 filters.
One of the nicest effects was when I stacked a warm-toned polarizer with a gold'n'blue polarizer to give some zest to some moving water over some rocks in brutally flat light.
I'll also stack a polarizer or colour intensifier with a GND and ND or basically any combination of them up to three.
The only problem with stacking my polarizers is there is an overall magenta cast. It's easy enough to fix in digital by adjusting the WB, but on film it would be more of a problem.
My two polarizers stacked:
Colour Intensifier, 3 stop soft edge GND & a Neutral Density filter (IIRC)