For 2008R2 and above. You should be able to add your additional dataset as a secondary axis. I am not at machine that has SSRS, but I believe you just right click on the values set, go to properties, and then select secondary for the axis.

This blog may help get your started.

http://sqlservermsbiblog.blogspot.com/2010/12/secondary-axis-in-ssrs-chart.html