<script> 
function test()
{ 
  var element_1 = document.getElementById('ch1');
  var element_2 = document.getElementById('ch2');
  
  var textbox_1 = document.getElementById('txt1');
  var textbox_2 = document.getElementById('txt2');
  
  if (element_1.checked == true)
  {
    textbox_1.value = element_1.value;
  
    if (element_2.checked == true)
    {
      textbox_2.value = element_2.value;
    } else {
      textbox_2.value = '';
    }
  } else {
    textbox_2.value = '';
  
    if (element_2.checked == true)
    {
      textbox_1.value = element_2.value;
    } else {
      textbox_1.value = '';
    }
  }
} 
</script> 

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