I thought that would be too easy of an answer! :-)
I have several other suggestions, then. Hopefully one of them will work for you.
Or if you prefer a callback and are using the RadAjaxManager, you can fire off an AjaxRequest and handle it on the server
// ramMain is a reference to your RadAjaxManager
ramMain.AjaxRequest += RadAjaxManager_AjaxRequest;
sender, AjaxRequestEventArgs e)
// Populate secondary dropdown
Another option is to put a button in the FooterTemplate that the user clicks on when all selections are made. If it's an ImageButton with a nice graphic it could be quite intuitive for your users.
"- Done -"