Wpf ComboBox Problem

Hey guys.

I have two ComboBoxes both have exactly the same Items. My Problem ist that I want to select an item in the first ComboBox, which should be deleted in the second. But it doesn't work.

Die Events:

^{private void cb_StoneCoordinates1_SelectionChanged(object sender, SelectionChangedEventArgs e)}
^{
^{//cb_StoneCoordinates2.Items.RemoveAt(cb_StoneCoordinates1.Items.IndexOf(cb_StoneCoordinates1.SelectedItem));}
^{cb_StoneCoordinates2.Items.RemoveAt(cb_StoneCoordinates1.SelectedIndex);}

^}

^{private void cb_StoneCoordinates2_SelectionChanged(object sender, SelectionChangedEventArgs e)}
^{
^{//cb_StoneCoordinates1.Items.RemoveAt(cb_StoneCoordinates2.Items.IndexOf(cb_StoneCoordinates2.SelectedItem));}
^{cb_StoneCoordinates1.Items.RemoveAt(cb_StoneCoordinates2.SelectedIndex);}
^}

^{Hier noch ein Bild meiner Andwendung, http://tempuri.org/tempuri.html}