How do I establish a secondary axis in Numbers on an iPad Pro?
I did figure this out, sort of.  In some configuration, after you make the chart, there is an opportunity to select chart type and see a list.  You can select a two axis chart. Then you have to find your way to the menu that lets you edit the series and edit them one by one to have the axis you want.  

I find this all very non-intuitive. After solving these problems I can’t remember what the sequence was.  The sequence of actions that seems natural tends to go nowhere.  Would love to move to using Numbers primarily but the entire option selection system needs to be reworked, at least on the iPad.