<form>
  <input type="checkbox" name="left_block" value="1111">
  <input type="checkbox" name="left_block" value="2222">
  <input type="checkbox" name="left_block" value="3333">
  <input type="checkbox" name="left_block" value="4444">
  <br>
  <input type="text" name="txt1" id="txt1">
  <input type="text" name="txt2" id="txt2">
  <br>
</form>
<script> 

(function(bind)
{
  var

  form  = document.forms[0],

  ult    = form.txt1,
  penult = form.txt2,

  history = [];

  bind.call(form, 'click', function voodoo(e)
  {
    var node = e.target || e.srcElement, i, value = node.value, entries;

    if(node.name != 'left_block')
    {
      return;
    }

    if(node.checked)
    {
      history.push(value);
    }
    else
    {
      i = history.length;

      while(i--)
      {
        if(history[i] == value)
        {
          history.splice(i, 1);
        }
      }
    }

    entries = history.slice(-2)

    ult.value    = entries.shift() || '';
    penult.value = entries.shift() || '';

  });

})(document.addEventListener ?

function(type, fn, bubble)
{
  this.addEventListener(type, fn, !!bubble);
} :

function(type, fn)
{
  this.attachEvent('on' + type, fn);
});

</script>