functions.php

<?php

/**
 * Archive Grid Divider
 *
 */
function be_grid_divider() {
	// only run on blog home and archives
	if( !( is_home() || is_archive() ) )
		return;
		
	if( in_array( 'one-half', get_post_class() ) && !in_array( 'first', get_post_class() ) )
		echo '<div class="grid-divider"></div>';
}
add_action( 'genesis_after_post', 'be_grid_divider' );

style.css

.grid-divider {
	overflow:hidden;
	width: 100%;
	border-top: 1px solid #ccc;
}