ho risolto trovando un post sul form
codice:
<input type="radio" name="prova" value="uno" onClick="togglebis(this);">
<input type="radio" name="prova" value="due" onClick="togglebis(this);">
<div style="display:none;" id="sub1">sub1</div>
<div style="display:none;" id="sub2">sub2</div>
<script>
function togglebis(radio){
if(radio.value=='due'){document.getElementById('sub2').style.display='block'; document.getElementById('sub1').style.display='none';}
else if(radio.value=='uno'){document.getElementById('sub1').style.display='block';document.getElementById('sub2').style.display='none';}
}
</script>
grazie a tutti