title: Switching Binding Sources using a MultiBinding
body: <p>I have a DataBinding with a MultiBinding of two ObservableCollections, and i want to switch between them on a condition with a MultiConverter. 
  So the converter gives the right collection, but the binding doesn't seem to be updated.</p>
  
  <p>Any Ideas??</p>
  
  <p>Greets, </p>
  
  <p>Jürgen</p>