RadGridView for Silverlight does not support the exact case, i.e. you can reorder the columns inside or outside the parent common header group.
You can check our online demo
for an example.
Generally you can control how the columns will be reordered through the ReorderColumnsMode setting of RadGridView. Then, when the ColumnReordering event is raised you can cancel the requested reorder.
You can also check our documentation:
I hope this helps you to customize the reordering as you wish.
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