Problem isn't with the multiple selectedItem but with the single selecteditem.
In my case I have :
List of tabControls.
SubList in a datagrid (for each tabcontrols)
Select something in datagrid, I show details right side.
Haven't been able to make it work with your example yet, and I can't even attach the file anyway.
Will try again tomorrow.
In the mean time i was able to bypass the problem, by getting the selectedItem in the
RadGridView_SelectedCellsChanged method. Which works but was hoping to make it work directly with selecteditem to save code.