The following code snippet demonstrates how you can clone or copy all the nodes in TreeView1 to TreeView2 by clicking on Button1.

private void IterateTreeNodes( TreeNode originalNode, TreeNode rootNode )
{
  foreach ( TreeNode childNode in originalNode.Nodes )
  {
    TreeNode newNode = new TreeNode( childNode.Text );
    newNode.Tag = childNode.Tag;
    treeView2.SelectedNode = rootNode;
    treeView2.SelectedNode.Nodes.Add( newNode );
    IterateTreeNodes( childNode, newNode );
  }
}

// copy nodes from treeView1 to treeView2
private void button1_Click( object sender, EventArgs e )
{
  foreach ( TreeNode originalNode in treeView1.Nodes )
  {
    TreeNode newNode = new TreeNode( originalNode.Text );
    newNode.Tag = originalNode.Tag;
    treeView2.Nodes.Add( newNode );
    IterateTreeNodes( originalNode, newNode );
  }
}

Contributed from George Shepherd's Windows Forms FAQ