Additional sidebar region and contentwide variable

Hi,
first thanks for the great theme. Here's my question:
I have added a additional sidebar sidebar_left that should show up on some pages. On this pages the region sidebar_first is not necessary. The additional sidebar shows up on this specific sites, but the additional statement that i made in the page.tpl.php: if($page['sidebar_left']) { $contentwid= "eleven"; } else { $contentwid= "sixteen"; } seams not to be recognized. The content area still uses the class with sixteen columns and not the smaller column style eleven when the sidebar_first is true on a site. What i am missing here that the switch for the columns works with my additional region as it works with the original region sidebar_first? Thanks.