In order to reorder the Items of the RadTabControl you need to use MVVM approach and populate the items through the ItemsSource property. Once you have created the collection holding the business objects representing the TabItems you can reorder them by simply removing the items from the collection and inserting it in a different position in the same collection.
I have attached a sample project demonstrating the above approach for your convenience.
I finally solved this with another workaround: by creating two overlapping tab controls (one with 1 tab and the second one with 2) and used the Visibility property on the tab control.