Cacher is the code snippet organizer for pro developers

.sidebar-middle-left,
.sidebar-middle-right {
	display: inline;
	float: left;
	width: 212px;
}

.sidebar-middle-right {
	float: right;
}

<?php

add_action( 'genesis_after_sidebar_widget_area', 'child_split_sidebars' );
/**
 * In the primary sidebar, after the original widget area, split into two
 * widget areas, before continuing with a full-sidebar-width widget area.
 *
 * @since 1.0.0
 *
 * @author Gary Jones
 * @link http://code.garyjones.co.uk/genesis-split-sidebars/#comment-7098
 */
function child_split_sidebars() {

	?>
	<div class="sidebar-middle">

		<div class="sidebar-middle-left">
			<?php dynamic_sidebar( 'sidebar-middle-left' ); ?>
		</div><!-- end .sidebar-middle-left -->

		<div class="sidebar-middle-right">
			<?php dynamic_sidebar( 'sidebar-middle-right' ); ?>
		</div><!-- end .sidebar-middle-right -->

	</div><!-- end .sidebar-middle -->

	<div class="sidebar-bottom">
		<?php dynamic_sidebar( 'sidebar-bottom' ); ?>
	</div><!-- end .sidebar-bottom -->
	<?php

}

// Register the sidebars with WordPress
genesis_register_sidebar(
	array(
		'id'          => 'sidebar-middle-left',
		'name'        => __( 'Sidebar Middle Left', 'theme-text-domain' ),
		'description' => __( 'This shows up on the left side of the split sidebars.', 'theme-text-domain' ),
	)
);

genesis_register_sidebar(
	array(
		'id'          => 'sidebar-middle-right',
		'name'        => __( 'Sidebar Middle Right', 'theme-text-domain' ),
		'description' => __( 'This shows up on the right side of the split sidebars.', 'theme-text-domain' ),
	)
);

genesis_register_sidebar(
	array(
		'id'          => 'sidebar-bottom',
		'name'        => __( 'Sidebar Bottom', 'theme-text-domain' ),
		'description' => __( 'This shows up underneath the split sidebars.', 'theme-text-domain' ),
	)
);