您应该能够通过以下方式做到这一点：

/**
 * Setup a custom hook before the second post on the search page
 */
add_action( \'the_post\', function( $post, \\WP_Query $q )
{
    if( $q->is_search() && $q->is_main_query() && 2 === $q->current_post )
    {
        do_action( \'wpse_before_second_post_in_search\' );
    }
}, 10, 2 ); 

/**
 * Inject a Div after the first post on the search page
 */
add_action( \'wpse_before_second_post_in_search\', function()
{
    echo \'<div>My Injected Div</div>\';
} );