You would need to create an entire new PHP file and edit this line <?php include(TEMPLATEPATH."/l_sidebar.php");?>
to what you want.
So if you make a new side bar page and you want the index to be different from all the other pages, add <?php include(TEMPLATEPATH."/l_sidebar.php");?>
to your index.php folder.
Then on your page.php file (or whatever you call it) edit <?php include(TEMPLATEPATH."/l_sidebar.php");?>
to whatever your other sidebar is.
EXAMPLE:
Your new sidebar page is l_sidebar_2.php change <?php include(TEMPLATEPATH."/l_sidebar.php");?>
to <?php include(TEMPLATEPATH."/l_sidebar_2.php");?>
Make sure the measurements are the EXACT same, and you use the same DIV ID’s, or you will screw some stuff up.