If the volume of a gas (ozone) is 50 ppm in a fixed size sealed container, and an amount of outside air equal to the container's initial volume is mixed with the first volume in a separate sealed container, would the PPM be reduced to 25?

Depends on the ppm of ozone in the outside gas. I think. Surely if the volumes are identical and there is zero ppm of ozone in the outside gas then yes, 25ppm is the answer. It's a guess, I might say educated, but don't bank on it