Multiple Sidebars

I have two sidebars, one that I want to display on the index.php template and one for the post.php template. I am using the get_sidebar() command. The sidebar file name for the index.php template is sidebar.php, while the one for the post page is sidebar-post.php. I am using the following code to read out my sidebar-post.php sidebar:

<? get_sidebar('post'); ?>

However, this simply returns a page with no sidebar at all. What am I doing wrong? Thanks. And yes, I have widgets for the second sidebar.