All filters that I have seen so far (from DJI, Freewell, Polarpro, PGYTech) cover both cameras, It would be difficult to design something different and there is no reason to do so either.
Polarizing filters reduce the amount of haze in the atmosphere, present mostly when it's sunny and the temperature is increased. For example in a landscape, viewing mountains from afar. Under these conditions the particles in the atmosphere reflect the rays of light to the camera producing what we perceive as haze. Polarizer filters reduce this by only allowing light polarized at a specific angle to enter the lens. Another benefit is that contrast is increased as well.