I did a quick search
through our online resources and I came across a discussion related to the functionality you requested. Here is a custom hierarchical tab control solution that we created using RadTabStrip. It does not cover fully the functionality of the standard multi-row tab control, but can be very useful in some cases. It loads the tab information from a Xml file and it offers visual customization.
This functionality has also been added to our ToDo and we will start working on it as soon as we finish with the highest priority tasks.
the Telerik team