WordPress multi-column content

default theme = Twenty Eleven ?

start by creating a child theme to work with: http://codex.wordpress.org/Child_Themes

how much different?
– just the formatting or also the output ?
, such as different thum

if the difference is just in the formatting:
edit content.php in the child theme, and add some code in this line:

<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

for instance:

<article id="post-<?php the_ID(); ?>" <?php $full_half = ($wp_query->current_post == 0) ? 'full-width' : 'half-width'; post_class($full_half); ?>>

this will give you an additional css class for the post which you could use to format any other than the first post as half wide.