or
radRichTextBox1.Document.Selection.Clear();
DocumentTextSearch search = new DocumentTextSearch(radRichTextBox1.Document);
foreach (var textRange in search.FindAll("Section1"))
{
radRichTextBox1.Document.Selection.AddSelectionStart(textRange.StartPosition);
this.radRichTextBox1.Document.Selection.AddSelectionEnd(textRange.EndPosition);
// what to do here to insert the formattted text from radRichTextBox2
}