Partner metrics can be used to track the performance of two metrics together. This is particular useful in certain visualisations, such as Statement View.
In the Report Builder, click pair metrics (third icon along) to the right hand side of the selected metric.
From the dashboard, choose statement under visualisations. This will display a total for your metric and the paired metric will appear alongside it to the right.
If you look under visualisation settings for statement, you can change the priority of the paired metric to display % in comparison to the main metric.