Prova questo ciao
<html>
<head>
<script language="JavaScript">
function nascondi(val){
alert(val);
if(val=="r1"){
document.forms(0).c1.style.visibility="hidden";
document.forms(0).c2.style.visibility="visible";
}else if(val=="r2"){
document.forms(0).c2.style.visibility="hidden";
document.forms(0).c1.style.visibility="visible";
}//end if
}//end function
</script>
</head>
<body>
<form>
<center>
Clicca per vedere il funzionamento



<input type="radio" name="sel" value="r1" onclick="nascondi(this.value)"> radio 1

<input type="radio" name="sel" value="r2" onclick="nascondi(this.value)"> radio 2




<input type="checkbox" name="sel" id="c1" style="visibility:visible"> check 01

<input type="checkbox" name="sel" id="c2" style="visibility:hidden"> check 02

</center>
</form>
</body>
</html>