You can specify the admin event inside admin tag

...
  <admin>
     <events> 
        <sales_order_shipment_save_after> 
           <observers>
            <Namespace_Modulename_Customevent>
             <type>singleton</type>
             <class>Namespace_Modulename_Model_Observer</class> 
             <method>customFunction</method> 
            </Namespace_Modulename_Customevent>
          </observers>
       </sales_order_shipment_save_after> 
     </events>
  <admin>
...

You can specify the admin event inside admin tag

...
  <admin>
     <events> 
        <sales_order_shipment_save_after> 
           <observers>
            <Namespace_Modulename_Customevent>
             <type>singleton</type>
             <class>Namespace_Modulename_Model_Observer</class> 
             <method>customFunction</method> 
            </Namespace_Modulename_Customevent>
          </observers>
       </sales_order_shipment_save_after>  

        <sales_order_invoice_register> 
           <observers>
            <Namespace_Modulename_AnotherCustomevent>
             <type>singleton</type>
             <class>Namespace_Modulename_Model_Observer</class> 
             <method>anotherCustomFunction</method> 
            </Namespace_Modulename_AnotherCustomevent>
          </observers>
       </sales_order_invoice_register> 

     </events>
  <admin>
...

Like this you can use many events in you custom module