Finally managed to find the sollution. Instead of binding to separate CollectionViewSource, I'd named the first datagrid as x:Name="FList" and then used binding to ElementName=FList, Path=SelectedItem and then defined the Path to the needed property.
<StackPanel DataContext="{Binding ElementName=FList, Path=SelectedItem, Mode=TwoWay}" Name="StackPanel2" >
<TextBox Text="{Binding Path=ForumOptions.Country.CountryName}"/>
</StackPanel>