The TreeViewItem is itself an ItemsControl, it has its own Items collection and ItemContainerGenerator as well.
If you define your items in Xaml, then getting the TreeView item is as simple as getting the item and casting it:
var treeViewItem = otherTreeViewItem.Items as RadTreeViewItem;
Then, if you are binding the TreeView you will need to use the ItemContainerGenerator as you have done in your code sample.
the Telerik team