module.php

<?php

function MODULE_my_form($form, &$form_state) {
  // ...
  $form['#pre_render'][] = 'MODULE_my_form_add_table';
  return $form;
}

function MODULE_my_form_add_table($form) {
  
  $table_fields = array('one', 'two');
  
  $table = array(
    '#prefix' => '<table>',
    '#suffix' => '</table>',
  );
  foreach($table_fields as $name) {
    if(isset($form[$name])) {
      
      $table[$name] = array(
        '#prefix' => '<tr><td>',
        '#suffix' => '</td></tr>',
      ) + $form[$name];
      
      unset($form[$name]);
    }
  }
  
  $form['table'] = $table;
  
  return $form;
}