How do I go about having two view models communicate with one another using MVVM Light. I know how to use the messenger class and register etc.. Here is my Scenario

A Settings View ---> a Settings View Model 
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A MainPage View ---> A MainPage ViewModel

If something changes in the Settings View it will Message back to the Settings View Model. So then I want the Settings View Model to communicate to the MainPage View Model about what changed. THe MainPage ViewModel will then tell the View.