ho trovato qualcosa di molto simile
codice:
<script type="text/javascript">
function FillBilling(f) {
   if(f.billingtoo.checked == true) {
     f.billingname.value = f.shippingname.value;
     f.billingcity.value = f.shippingcity.value;
   }
 }
</script> 
<FORM name="frm">
<input type="checkbox" name="billingtoo" onclick="FillBilling(this.form)">
Mailing Address
 


 <form>
 40:
 <input type="text" name="shippingname">
 

 41:
 <input type="text" name="shippingcity">
 


 


 Billing Address
 


 40:
 <input type="text" name="billingname">
 

41:
 <input type="text" name="billingcity">
 </form>
solo che se dupplico i campi



40:
<input type="text" name="billingname">


41:
<input type="text" name="billingcity">



40:
<input type="text" name="billingname">


41:
<input type="text" name="billingcity">

non va +