Side by side blog posts that are tied together

A very simple solution: Add a second editor field to the post editor, and use the meta field content in a filter on the_content.

add_filter( 'the_content', 'wpse_77811_extra_content' );

function wpse_77811_extra_content( $content )
{
    return $content . get_post_meta($post->ID, '_t5_extra_box', TRUE );
}