<script type="text/javascript">
/*<CDATA[[*/
function g1(){
for (i = 0; i < document.forms[0].g2.length; ++i){
if(document.forms[0].g2[i].ckecked == true){
document.forms[0].g2[i].checked = false;
}
}
}
function g2(){
for (i = 0; i < document.forms[0].g1.length; ++i){
if(document.forms[0].g1[i].checked == true){
document.forms[0].g1[i].checked = false;
}
}

}
/*]]>*/
</script>
</head>
<body>
<form action="" method="" enctype="application/x-www-form-urlencoded" >
<b>Group 1</b><br />
<input type="radio" name="g1" onclick="g1()" />
<input type="radio" name="g1" onclick="g1()" /><br />
<b>Goutp 2</b><br />
<input type="radio" name="g2" onclick="g2()" />
<input type="radio" name="g2" onclick="g2()" />
</form>

By checking either of the radio button, the rest should be un-checked...