Submitting fieldsets
You can use the #tree property on both fieldsets, e.g.

$form['fieldset_1'] = array(
  '#type' =&gt; 'fieldset',
  '#title' =&gt; t('Title 1'),
  '#tree' =&gt; TRUE,
);

$form['fieldset_1']['name'] = array(
  '#type' =&gt; 'textfield',
  ...
);

$form['fieldset_2'] = array(
  '#type' =&gt; 'fieldset',
  '#title' =&gt; t('Title 2'),
  '#tree' =&gt; TRUE,
);

$form['fieldset_2']['name'] = array(
  '#type' =&gt; 'textfield',
  ...
);