I think because you Bind the selectedValue to the TeamName.
SelectedValue="{Binding Path=TeamName}"
If you remove the bind, they probably work. I think whats happen here is, when you change the selection in one combobox, the value of the new selected item is written in the current items-name. So it looks that the first combobox does the same, but if you open it, you'll see that value twice.