This is all a bit baffling and ass backwards.

The OP wants extra contrast, which is normally dealt with by choices of film>exposure>developer>time and dilution. But he is using a yellow filter for contrast and stacking it with a polarizer to block some light. Stacking two filters will increase the possibility of flare, which will reduce contrast. Unless a polarizer is constantly tweaked to bring back highlights it is at best random in effect, at worst it will reduce the mid tone/micro contrast by removing highlights and reflections. If the polarizer is constantly tweaked it can only mean the lens hood isn't being used (unless a slow working formal portrait session perhaps), which will reduce contrast. So for the sake of buying an ND filter of the right strength, and developing the film for extra contrast, every action taken is mitigating against increasing contrast.

Steve