How to update viewmodel based on selected item of listview present in other viewmodel on same page?

I have a page with master viewmodel.This mater viewmodel is having two different view models with which two content views(custom user control) are binded. The first viewmodel A has a listview in it.On item tap command I want to refresh the content of Viewmodel B. Both A and B are part of same page and I have to achieve this using viewmodels only(no page behind calls).
Till now I am able to populate "A" but on item tap(selecting an item), I am not able to refresh the other viewmodel "B".
Any help will be appreciated