RadTreeList indeed does not have a SelectedValue property, so you would need a different approach. Having in mind the two controls are in different user controls a server-side approach would rather be needed.
You could make the treelist postback on item click RadTreeList1.ClientSettings.AllowPostBackOnItemClick = "true" and in the ItemCommand event save the clicked row value in Session. Clear this value on each Page_Load event of this same user control.
Then, in the PreRender event of the other user control check if there is such value and if so, modify the select parameter of the datasource control accordingly.
Finally, call Rebind() for the grid.
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