There is no way to display different metrics on the performance compare drill down page.
You can have multiple metrics in the plugin on a dashboard though.
- Scope to the servers that you need
- Choose your first metric for the 2012 servers
- Click advanced, add your second metric for the 2008 servers after the first separated by a comma
Both lines will appear on the page. You can expand the area by dragging the orange line at the bottom of the section. I also find that setting the data range to ‘fit to data‘ helps too.