This behavior is not supported with the current version of RadTimeline for WPF. Basically, the control arranges every item in the first available free row. Probably you can achieve this result to some extent, in case you use a separate timeline control for every group - that is, one timeline for Group1, another for Group2, etc. Then, use the GroupPath to arrange the items in specific rows - one row represents one group of items.
Additionally, you can review our GanttView control to see if it better fits your requirements.
On a side note, could you please share more detailed information regarding your scenario. We might be able to suggest another control for your case, or add this as feature request to our ToDo list.
the Telerik team