Split Sidebar in Genesis - http://gamajo.com/split-sidebar-genesis

functions.php

<?php

// Don't include the <?php above

add_action( 'after_setup_theme', 'prefix_register_split_sidebars', 5 );
/**
 * Register the two widget areas that make up our visually separated sidebar.
 *
 * @author Gary Jones
 * @link   http://gamajo.com/split-sidebar-genesis
 */
function prefix_register_split_sidebars() {
	genesis_register_sidebar(
		array(
			'id'          => 'prefix-split-sidebar-left',
			'name'        => __( 'Primary Sidebar Bottom Left', 'textdomain' ),
			'description' => __( 'This shows up underneath the primary sidebar.', 'textdomain' ),
		)
	);

	genesis_register_sidebar(
		array(
			'id'          => 'prefix-split-sidebar-right',
			'name'        => __( 'Primary Sidebar Bottom Right', 'textdomain' ),
			'description' => __( 'This shows up underneath the primary sidebar.', 'textdomain' ),
		)
	);
}

add_action( 'genesis_after_sidebar_widget_area', 'prefix_display_split_sidebars' );
/**
 * Add two sidebars underneath the primary sidebar.
 *
 * @since 1.0.0
 *
 * @author Gary Jones
 * @link   http://gamajo.com/split-sidebar-genesis
 */
function prefix_display_split_sidebars() {
	?>
	<div class="split-sidebars">
		<?php
		genesis_widget_area( 'prefix-split-sidebar-left' );
		genesis_widget_area( 'prefix-split-sidebar-right' );
		?>
	</div>
	<?php
}

style.css

.split-sidebars > .widget-area {
	float: left;
	width: 50%;
}