fatto,funziona,grazie mille 
codice:
<script type="text/javascript">
function show(str){
var ciccio = <%=nufo%>
for (i=1;i<=ciccio;i++){
document.getElementById(str).style.display = 'block';
if (str=='quant'+i) str='stock'+i; else str='quant'+i;
document.getElementById(str).style.display = 'none';
}
}
</script>
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<input type="radio" name="rd<%=nfor%>" value="0" onclick="show('quant<%=nfor%>');">
<div id="quant<%=nfor%>">
<input name="sconto_quant<%=nfor%>" onkeyup="this.value=this.value.replace(/[^0-9\.\,]/gi, '');" type="text" value="0" size="3" maxlength="8" class="form"> %
<input name="pezzi_quant<%=nfor%>" onkeyup="this.value=this.value.replace(/[^0-9\.\,]/gi, '');" type="text" value="0" size="3" maxlength="8" class="form"></div>
<input type="radio" name="rd<%=nfor%>" value="1" onclick="show('stock<%=nfor%>');">
<div id="stock<%=nfor%>">
<input name="sconto_stock<%=nfor%>" onkeyup="this.value=this.value.replace(/[^0-9\.\,]/gi, '');" type="text" value="0" size="3" maxlength="8" class="form"> %
<input name="pezzi_stock<%=nfor%>" onkeyup="this.value=this.value.replace(/[^0-9\.\,]/gi, '');" type="text" value="0" size="3" maxlength="8" class="form">
</div>