How to use input filtering on node type with 2 textareas

Hi,

I have created a module which defines 2 textareas (see code below). I want to add input filtering to the textareas, but this doesn't seem to work.
No matter if i set the input format for a textarea to 'Filtered HTML' or not, all tags are saved identical to the input text, which contains tags that are not allowed.

Because i have 2 textareas, i had to set the $parent attribute for filter_form(), to specify which filter applies to which textarea.
Note that what i want is 2 fieldsets, each containing a textarea, and input formatting options.

Here is my implementation of hook_form():

... some other form items
  $form['left_field'] = array(
    '#type' => 'fieldset',
    '#collapsible' => FALSE,
    '#collapsed' => FALSE,
    '#attributes' => array('class' => 'left_container'),
  );
  
  $form['left_field']['left_format']['left_content'] = array(
    '#type' => 'textarea',
    '#title' => 'Left column',
    '#rows' => 20,
    '#cols' => 60,
    '#required' => TRUE,
    '#default_value' => $node->left_content,
  );  
  $form['left_field']['left_format']['format'] = filter_form($node->left_format, NULL, array('left_field', 'left_format', 'left_content'));    
  
  $form['right_field'] = array(
    '#type' => 'fieldset',
    '#collapsible' => FALSE,
    '#collapsed' => FALSE,
    '#attributes' => array('class' => 'right_container'),
  );
  
  $form['right_field']['right_format']['right_content'] = array(
    '#type' => 'textarea',
    '#title' => 'Right column',
    '#rows' => 20,
    '#cols' => 60,
    '#default_value' => $node->right_content,
  );  
  $form['right_field']['right_format']['format'] = filter_form($node->right_format, NULL, array('right_field', 'right_format', 'right_content'));  
  return $form;

My DB fields are set up like this:

        `nid` INT UNSIGNED NOT NULL DEFAULT '0',
        `vid` INT UNSIGNED NOT NULL DEFAULT '0',
        `left_content` LONGTEXT NULL ,
        `left_format` INT( 10 ) UNSIGNED NOT NULL DEFAULT '0',
        `right_content` LONGTEXT NULL ,
        `right_format` INT( 10 ) UNSIGNED NOT NULL DEFAULT '0',
        PRIMARY KEY (nid, vid),

The $node->left_format & $node->right_format contain the right numeric representation of the selected filter, while $node->format is 0.
What could i do to filter the input in the textarea according to the selected format?

Any help on this one?

Thanks a lot!