Wordpess plugin that adds an extra post meta box for a short (or just additional) excerpt. Bung it in your wp-content/plugins directory, get the short excerpt using : get_post_meta($post->ID, "ape_short_excerpt", true) ) :

EnigmaticApe_ShortExcerpts_WP_Plugin.php

<?php
/*
Plugin Name: Add Short Excerpt Meta
Plugin URI: https://gist.github.com/enigmaticape/4751121
Description: Add an additional excerpt box for front page
Version: 1.0
Author: Steve Trewick
Author URI: http://www.enigmaticape.com
License: Public Domain
*/
    
    
  function ape_save_post_class_meta( $post_id, $post ) {

    // Verify nonce, user perms
    if ( !isset( $_POST['ape_short_nonce'] ) || 
       !wp_verify_nonce( $_POST['ape_short_nonce'],basename( __FILE__ )))
       return $post_id;
      
    $post_type = get_post_type_object( $post->post_type );
      
    if ( !current_user_can( $post_type->cap->edit_post, $post_id ) )
      return $post_id;


    
    $new_meta_value = (isset( $_POST['ape-post-class'] ) ? $_POST['ape-post-class'] : '');
    $meta_key       = 'ape_short_excerpt';
    $meta_value     = get_post_meta( $post_id, $meta_key, true );

    // CREATE
    if ( $new_meta_value &&  ('' == $meta_value) ) {
        add_post_meta( $post_id, $meta_key, $new_meta_value, true );
    } // UPDATE
    elseif ( $new_meta_value && ($new_meta_value != $meta_value) ) {
        update_post_meta( $post_id, $meta_key, $new_meta_value );
    } // DELETE
    elseif ( ('' == $new_meta_value) && $meta_value ) {
        delete_post_meta( $post_id, $meta_key, $meta_value );
    }
  }



  function ape_short_excerpt_class_meta_box( $object, $box ) { 

    $label_txt = _e( "<p>Short excerpt to be displayed on front page.</p>", 'example' );

    echo wp_nonce_field( basename( __FILE__ ), 'ape_short_nonce', true, false );
    echo '<label for="ape-post-class">'. $label_txt . '</label>';
    echo '<textarea name="ape-post-class"'.
                  ' class="widefat"'.
                  ' id="ape-post-class"'.
                  ' rows="6"'.
                  ' cols = "40">'.
                  esc_attr( 
                    get_post_meta( $object->ID, 'ape_short_excerpt', true ) 
                  ) . 
          '</textarea>';
  }

  function ape_add_post_meta_boxes() {

      add_meta_box(
          'ape-post-class',
          esc_html__( 'Short Excerpt' ),
          'ape_short_excerpt_class_meta_box',
          'post', 
          'normal',                               
          'default'
      );
  }
    
    function ape_short_excerpt_meta_boxes_setup() {
        add_action( 'add_meta_boxes', 'ape_add_post_meta_boxes' );
        add_action( 'save_post', 'ape_save_post_class_meta', 10, 2 );
    }

    add_action( 'load-post.php', 'ape_short_excerpt_meta_boxes_setup' );
    add_action( 'load-post-new.php', 'ape_short_excerpt_meta_boxes_setup' );


?>