<script> 
function test(Id)
{ 
  var element = document.getElementById(Id);
  
  var textbox_1 = document.getElementById('txt1');
  var textbox_2 = document.getElementById('txt2');
  
  if (element.checked == true)
  {
    if (textbox_2.value.length > 0)
    {
      textbox_2.value = element.value;
    } else {
      if (textbox_1.value.length > 0)
      {
        textbox_2.value = element.value;
      } else {
        textbox_1.value = element.value;
      }      
    }
  } else {
    if (textbox_2.value.length > 0)
    {
      textbox_1.value = textbox_2.value;
      textbox_2.value = '';
    } else {
      textbox_1.value = '';
    }
  }
} 
</script> 

<input type="checkbox" name="left_block" id="ch1" value="1111" onClick="javascript:test('ch1');"> 
<input type="checkbox" name="left_block" id="ch2" value="2222" onClick="javascript:test('ch2');"> 
<input type="checkbox" name="left_block" id="ch3" value="3333" onClick="javascript:test('ch3');"> 
<input type="checkbox" name="left_block" id="ch4" value="4444" onClick="javascript:test('ch4');"> 
<br> 
<input type="text" id="txt1" value=""> 
<input type="text" id="txt2" value=""> 
<br>