[jQuery] select the last 2 elements in list

[jQuery] select the last 2 elements in list


e.g.
<ul>
<li>a</li>
<li>b</li> <-
<li>c</li> <-
</ul>
I did:
<script type="text/javascript">
        $(document).ready(function() {
         $("#update li:last").addClass("last_li");
         $(jQuery.sibling("#update
li:last").prev).addClass("sec_last_li");
        });
</script>
The last element works, but not the 2nd last....