When you edit a quick table calculation and click "Customize" Tableau will create a calculated field that you can customize. I did this with your workbook and customized the two calculations to take into account the parameter selection. That was the main part of the solution.
I cleaned up a little by hiding the header of one of the axis, fixing the axis, and creating another calculated field to give the axis a dynamic label.
I've attached the workbook so you can see the results.
See the attached workbook with a solution to your requirement.
I did a small variation to your workbook
I created two Calculated field: % of Total Volume and % Running Total of Volume to configure the dual axis chart with two measures. Note that they are similar to the Table calculation you did using just one measure Volume
I included a logic in the calculation of the above mentioned calculated field where if Market Share (%) is selected the % Running Total is assigned Null values. This way that measure is remove from the chart.
The same approach was apply to % of Total Volume.
Take a look at the workbook, worksheet My Dual Axis
I hope this helps
Just what I needed. Many thanks Joshua - much appreciated
Thanks Ramon. A sightly different calc from Joshua and extremely helpful also. Much appreciated!