Check out this example : http://demos.telerik.com/silverlight/#Chart/MultipleYAxes
Basically, this is how an additional Y axis is added:
Evgeni "Zammy" Petrov
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items