The key is that you must add the all radiobuttons to your radajaxmanager updated controls. somthing like this

ajaxManager.AjaxSettings.AddAjaxSetting(rbtn1, ddl, pnlLoading);
ajaxManager.AjaxSettings.AddAjaxSetting(rbtn1, rbtn2, null);
ajaxManager.AjaxSettings.AddAjaxSetting(rbtn1, rbtn1, null);
ajaxManager.AjaxSettings.AddAjaxSetting(rbtn2, ddl, pnlLoading);
ajaxManager.AjaxSettings.AddAjaxSetting(rbtn2, rbtn2, null);
ajaxManager.AjaxSettings.AddAjaxSetting(rbtn2, rbtn1, null);

This will solve the problem. because after every postback all the dropdowns must updated.

Add following code to your radmenu control. replace targetfieldname with your field.

<DataBindings>

<telerik:RadMenuItemBinding TargetField="TargetFieldName" />

</DataBindings>