You have to make next changes:

1. In file "components/com_jshopping/js/functions.js" in function validateCheckoutAdressForm
after
    if (register_field_require.email){
        arrayId[i] = 'email';
        arrayType[i] = 'em';
        arrayParams[i] = '';
        arrayErrorMessages[i] = '';
        i++;
    }

add

    if (register_field_require.email2){
        arrayId[i] = 'email';
        arrayType[i] = 'eqne';
        arrayParams[i] = 'email2';
        arrayErrorMessages[i] = '';
        i++;
    }

2.     In file "components/com_jshopping/templates/default/checkout/adress.php"
after
        <?php if ($config_fields['email']['display']){?>
        <tr>
          <td class="name">
            <?php print _JSHOP_EMAIL ?> <?php if ($config_fields['email']['require']){?><span>*</span><?php } ?>
          </td>
          <td>
            <input type = "text" name = "email" id = "email" value = "<?php print $this->user->email ?>" class = "inputbox" />
          </td>
        </tr>
        <?php } ?>
       
add

        <?php if ($config_fields['email2']['display']){?>
<tr>
          <td class="name">
            <?php print _JSHOP_EMAIL2 ?> <?php if ($config_fields['email2']['require']){?><span>*</span><?php } ?>
          </td>
          <td>
            <input type = "text" name = "email2" id = "email2" value = "" class = "inputbox" />
          </td>
        </tr>
        <?php } ?> 

3. in file     "components/com_jshopping/lib/default_config.php"
after
 
    $fields_client['address'][] = "email";

add
    $fields_client['address'][] = "email2";