ViewModelSample.java

public class SharedViewModel extends ViewModel {}

public class MasterFragment extends Fragment {
    private SharedViewModel model;
    public void onCreate(Bundle savedInstanceState) {
        model = ViewModelProviders.of(getActivity())
          .get(SharedViewModel.class); //<- 共通のviewmodel
    }
}

public class DetailFragment extends LifecycleFragment {
    public void onCreate(Bundle savedInstanceState) {
        SharedViewModel model = ViewModelProviders.of(getActivity())
          .get(SharedViewModel.class); //<- 共通のviewmodel
    }
}