<?php

function test_main_form($form, $form_state) {
  $form['test1'] = array(
    '#type' => 'textfield',
    '#name' => 'test1',
    '#ajax' => array(
      'callback' => 'my_callback_function',
      'wrapper' => 'my_target_area',
      'method' => 'replace',
      'event' => 'blur',
    ),
  );
  $form['my_target_area'] = array(
    '#prefix' => '<div id="my_target_area">',
    '#suffix' => '</div>',
  );
  return $form;
}

function my_callback_function($form, $form_state) {
  $form['my_target_area'] = array(
    '#prefix' => '<div id="my_target_area">',
    '#suffix' => '</div>',
  );
  $form['my_target_area']['result'] = array(
    '#type' => 'markup',
    '#markup' => 'You have selected: ' . $_POST['test1'] . '<br>',
  );
  $form['my_target_area']['test2'] = array(
    '#type' => 'textfield',
    '#name' => 'test2',
    '#ajax' => array(
      'callback' => 'my_callback_function',
      'wrapper' => 'my_target_area',
      'method' => 'replace',
      'event' => 'blur',
    ),
  );
  return $form['my_target_area'];
}