title: fill ComboBox with items from different ViewModel
body: <p>I have a ComboBox binded to DataContext SceneViewModel, but I want to fill it with data from an observableCollection from another ViewModel called GearViewModel.
  How do I do this? or is this possible.</p>
  
  <p>Here is the xaml</p>
  
  <pre><code>&lt;UserControl x:Class="MoviePrepper.View.SceneView"
   DataContext="{Binding SceneViewModel, Source={StaticResource Locator}}"&gt;
  
  &lt;Grid&gt;
      &lt;ComboBox ItemsSource="{Binding to observableCollection in GearViewModel}}" SelectedItem="{Binding SceneCollectionView/Equipment, UpdateSourceTrigger=PropertyChanged}"/&gt;
  &lt;/Grid&gt;
  &lt;/UserControl&gt;
  </code></pre>