[monolithed]

не работает

плохо читали!

<input type="checkbox" name="" />
<input type="checkbox" name="" disabled="disabled" />

<script type="text/javascript">
window.onload = function(){
    var input = document.getElementsByTagName('input'); 
    input[0].onclick = function(){
        input[1].disabled = (input[1].disabled == 0) ? 1 : 0;
    };
};
</script>

или

<input type="checkbox" name="input" />
<input type="checkbox" name="input" disabled="disabled" />

<script type="text/javascript">
window.onload = function(){
    var input = document.getElementsByName('input');
    input[0].onclick = function(){
        input[1].disabled = (input[1].disabled == 0) ? 1 : 0;
    };
};
</script>