Two comboboxes with the same CollectionViewSource ItemSource update each other
This is normal, CollectionViews have a CurrentItem and if the ItemsSource is a CollectionView they get synchronized, see IsSynchronizedWithCurrentItem:
true if the SelectedItem is always synchronized with the current item in the ItemCollection; false if the SelectedItem is never synchronized with the current item; null if the SelectedItem is synchronized with the current item only if the Selector uses a CollectionView. The default value is null.
So you can just turn it off manually by setting that property to false.
(By the way, you can also bind to the CurrentItem of a CollectionView via a slash. e.g. People/Name binds to the Name property of the current person in People.)