errata corrige lo script era questo:
<script>
function cambia() {
y=document.form.radioB;
for(i=0;i<y.length;i++){
if(y[i].checked)document.forms["form"+i].style.visibility='visible'
else document.forms["form"+i].style.visibility='hidden';
}
}


</script>

in pratica al check del radio button si visualizza il campo nascosto:

<form name="form">
<input type=radio onclick="cambia()" name=radioB >Nuova Richiesta

<input type=radio onclick="cambia()" name=radioB>Modifica
</form>

<form name="form0" style="visibility:hidden;position:absolute" >
<input type="submit" value="Invia nuova richiesta" class=bottone name="irichiesta">
</form>

<form name="form1" style="visibility:hidden;position:absolute" >
<input type="submit" value="Invia nuova richiesta" class=bottone name="irichiesta">
</form>