Please refer to the following demo
demonstrating the different Load On Demand scenarios. Also please refer to the following demo
showing a scenario with related comboboxes.
In your particular case you should set text and value to the first one of the related comboboxes on page load. In case you want load data in the second combobox based on the value set in the first one you should subscribe on the OnClientLoad event and use the approach from the second demo in order to initiate a request for items.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed