yes you can do this, It will be very easy IF you want the sidebars on PAGEs too
First create a child theme of twenty-eleven then copy single.pgp and page.php to it
Edit single.php and change the last line from
<?php get_footer(); ?>
to
<?php get_sidebar(); ?>
<?php get_footer(); ?>
Do the same to page.php
Edit style.css and remove the following lines (should be about line 100 on)
/* Singular */
.singular #primary {
margin: 0;
}
.singular #content,
.left-sidebar.singular #content {
margin: 0 7.6%;
position: relative;
width: auto;
}
.singular .entry-header,
.singular .entry-content,
.singular footer.entry-meta,
.singular #comments-title {
margin: 0 auto;
width: 68.9%;
}
/* Attachments */
.singular .image-attachment .entry-content {
margin: 0 auto;
width: auto;
}
.singular .image-attachment .entry-description {
margin: 0 auto;
width: 68.9%;
}
Document what you did…just in case.
if you only want the sidebar on the single posts pages and not page pages, you are going to have to play with the CSS some more.