The following piece of code will allow you to have mutiple ASP.NET 2.0 Ajax CollapsiblePanelExtender controls on a single page and only one of them will be expanded at a time.  You can use this to keep the page length short.  You must set the BehaviorId of the CollapsiblePanelExtender, that is what the $find javascript function looks for.  Make sure to add all the BehaviorIds to the array in the pageLoad function.

<script type="text/javascript">

// WWB: Handles Mutual Exclusive Expansion.  Only One Extender Panel 
// Can Be Open At A Time

var extenders = [];
    
function pageLoad(sender, args)
{
    extenders[0] = $find("PanelExtender1");
    extenders[1] = $find("PanelExtender2");
    extenders[2] = $find("PanelExtender3");


    // WWB: Hook All The Extenders, If extenders[i] is null then
    // the extenders is not being displayed -- i.e. not visible
    for (var i=0; i< extenders.length; i++)
        if (extenders[i] != null)
            extenders[i].add_expandComplete( expandHandler );
}

function expandHandler(sender, args)
{
    for (var i=0; i< extenders.length; i++)
    {
        if ((extenders[i] != null) && (extenders[i] != sender))
            extenders[i]._doClose(); 
    } 
}
</script>