How to change the dependency property in nested widget?

Question

I use (WPF) synchronized scroll viewer:

http://www.codeproject.com/KB/WPF/ScrollSynchronization.aspx?msg=3577911#xx3577911xx

The main point, I have two datagrids for which I would like to set synchronized scroll viewers. XAML code:

<DataGrid AutoGenerateColumns="false" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" 
          Name="dataGrid1" GridLinesVisibility="None" >
  <DataGrid.Resources>
    <Style TargetType="ScrollViewer">
      <Setter Property="scroll:ScrollSynchronizer.ScrollGroup" Value="Group1" />
    </Style>
  </DataGrid.Resources>
</DataGrid>

The second one is identical (except the name). Now, the problem is, I populate the datagrid (its cells) with textboxes which have scrollbars as well, and by default they all set the same group. Meaning that for example I have 2000 scrollviewers in the group instead of 2.

So the question is -- how to set a different group for scrollviewers for textboxes? I tried this (it has to be in C# code):

        int i = 0;

        foreach (var db_col in query.FieldNames)
        {
                var template = new DataTemplate();
                var elemFactory = new FrameworkElementFactory(typeof(TextBox));

                // resetting the scrollviewer group ?
                elemFactory.SetValue(ScrollSynchronizer.ScrollGroupProperty, "");

                template.VisualTree = elemFactory;

                var col = new DataGridTemplateColumn();
                col.CellTemplate = template;
                col.Header = db_col;
                grid.Columns.Add(col);

                ++i;
        }

but it does not work. So -- how to set this property for the widget on its own, and not inherit it from the parent?

Remark 1: I changed the original code of synchronized scrollviewer, so now it accepts empty strings as group names.

Remark 2: what is completely odd, when I change the way this property is registered, and set option Inherits to false, it is inherited anyway!