Your DataContext is bound with the Default binding Mode (OneWay), try this :
<bb:FolderPicker DataContext="{Binding Path=SomeStringProperty, Mode=TwoWay}">
</bb:FolderPicker>
Vko
<bb:FolderPicker DataContext="{Binding Path=SomeStringProperty, Mode=TwoWay}">
</bb:FolderPicker>
Vko
