Hello Megan Vee,
Unfortunately such scenario is not supported by RadGridView. You could sort groups either by key or by an aggregate function (but not by a couple (or more) aggregate functions) as shown in the referenced example. Generally any change to group descriptor will invoke re-group of all items, so if you add aggregate and change GroupSortExpression everything should work. Of course there is another option to create custom aggregate function.
Let me know if there is something unclear.
All the best,
the Telerik team
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