There is only 1 dial, but 2 markers.
With the left, movable, marker you mark the ASA value on the dial. Then you can twist the dial along the right zero-marker, if you want temporarily change the exposure metering.
With all cameras one can change the ISO setting throughout the film.
At cameras with more basic controls (e.g. AE-1), this is just the only means for that change at exposere metering. However at these cameras one then may loose memory of what the actual ISO setting was. Thus at some cameras these two functions are parted (e.g. A-1).