I am not sure if I understand your scenario correctly. Could you please provide more detailed information or better yet send a small but runnable project that illustrates your current implementation? This way I would be able to investigate your specific case in details and advice you further.
On a side note, I will suggest to check this help topic
which covers the cascading functionality (even that it is for ComboBox it is applicable to the DropDownList too).
the Telerik team
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