Is the issue that you cannot show the % as a value on the second visualization? Depending on what version of Tableau you are using, you can drag the value over to the 'label' area of the marks shelf.
I think the issue with your first screen shot is that the scale for your line graph is off. You can edit that on the axis itself.
This blog post shows a pretty similar problem. Excel: How to create a dual axis chart with overlapping bars and a line
I've attached a workbook with an adjusted solution, showing a % for the line instead of units. Hopefully this fixes your issue.