Different sidebar shown on different pages

Hello,

I am trying to get WordPress to display a sidebar on single posts, and a different sidebar everywhere else on the site. If one of the sidebars has no widgets, I want it to go to the default sidebar configuration.

Currently my sidebar.php file has:
<?php if ( function_exists('dynamic_sidebar') && dynamic_sidebar(1) ) : else : ?>

and I’m trying to make it into:
<?php if ( function_exists('dynamic_sidebar') && dynamic_sidebar(2) ) : else : ?>
when on a single post.

I’ve tried many things, but I can’t seem to get it to work right. So far my code looks something like:

<?php if (is_single()) {
if ( function_exists('dynamic_sidebar') && dynamic_sidebar(2) ) : else :
} else (if ( function_exists('dynamic_sidebar') && dynamic_sidebar(1) ) : else :
}
?>

<li><h2><?php _e('Categories'); ?></h2>
<ul>
	<?php wp_list_cats('sort_column=name&optioncount=1&hierarchical=1'); ?>
</ul>
....

<?php endif; ?>

But this doesn’t seem to work! Any help will be appreciated.