Here's how to do it.

1. Go to ‘My Data’ and click on the table that you want to update and then on the ‘Chart designer’.
2. If you’ve already created a chart and want to change it, click on the chart.  Alternatively, click on ‘Create New Chart’ and create your chart.
3. Once your chart is created, click on the ‘Settings’ button next to the name of the column for which you wish to add a second Y axis. 
4. A pop-up box will open.  Click on the ‘Show on a 2^nd Y-axis on the RHS’ button and click on ‘Update’.  Click on the ‘Finished’ button when you’ve finished.

There is also a video available on the ‘Training’ page called “Creating a Secondary Axis / Scale”.  Click on the link to view it - https://www.targetdashboard.com/training/default.aspx?vidid=9