I don't have time to try to play with it right now, but you could try to mimic the process illustrated in the link to creating x-y axes in the earlier documention that shows the steps of adding the second axis on top of the first with the 'position' parameter and then moving axis position, etc.
With a polar, instead of setting the second y-axis location to 'right', the 'RAxisLocation' will have to be oriented somewhere besides the default 80 degree location, but since there's only one centroid for the axes, the two ticks at the center are inevitably going to be overlapping and I see no way around that in having competing +/-5 values on top of each other visually even if are in separate axes objects.
It seems to me also as a matter of the comparison, however you plot these comparatively it makes sense only if both are scaled to the same limit ranges.
