Almost forgotten, it’s actually a few years since I have used this plugin but still turn out te be usefull: Secondary html content wp plugin
As the name suggest it adds a second content area to the editor which can be handy at times.
Shame it has n’t been updated for quit some years nevertheless you can still download it here
<div class="left-sidebar-or-right-sidebar-or whatever"><?php echo the_secondary_content(); ?> </div>
If you intent on using this plugin you might want to replace one line. A user pointed out that this was causing an issue, read about it here.
Line 513 in the file secondary-html-content.php has to be changed from:
$this->WP_Widget( 'secondary-html-content', __( 'Secondary HTML Content', 'secondary_html_content' ), $widget_ops );to
parent::__construct( 'secondary-html-content', __( 'Secondary HTML Content', 'secondary_html_content' ), $widget_ops );