how to get radcombobox value using jquery

 The reason you cannot access the other controls on the page, is because the RadComboBox performs an Async request for the items and as such the other controls on the page are not accessible.

Try handling the OnClientItemsRequesting event, making use of the context object (that is passed to the server-side code) to send the selected value of the first combo.

Markup

<telerik:RadCodeBlock ID="RadCodeBlock" runat="server">

    <script type="text/javascript">

        function OnClientItemsRequesting(sender, eventArgs) {

            var comboBox1 = $find('<%= comboBox1.ClientID %>');
            var value = comboBox1.get_value();

            var context = eventArgs.get_context();
            context["ComboBox1Value"] = value;
        }

    </script>

</telerik:RadCodeBlock>

<telerik:RadComboBox ID="comboBox1" runat="server" 
    MarkFirstMatch="False">
    <Items>
        <telerik:RadComboBoxItem Text="Item 1" Value="0" />
        <telerik:RadComboBoxItem Text="Item 2" Value="1" />
    </Items>
</telerik:RadComboBox>

<telerik:RadComboBox ID="comboBox2" runat="server" 
            EnableLoadOnDemand="True" 
            MarkFirstMatch="False" 
            onitemsrequested="comboBox2_ItemsRequested"
            OnClientItemsRequesting="OnClientItemsRequesting">
</telerik:RadComboBox>            

Code behind

protected void comboBox2_ItemsRequested(object sender, Telerik.Web.UI.RadComboBoxItemsRequestedEventArgs e)
{
    string selectedValue = e.Context["ComboBox1Value"].ToString();
}

Hope this helps.