there are some errors in the script syntax and the form referances are backwards, try this instead:
<script type="text/javascript">
<!--
function shipsame(form){
if(form.sameasbilling.checked){
form.second_address.value = form.address.value;
form.second_address2.value = form.address2.value;
form.second_city.value = form.city.value;
form.second_state.value = form.state.value;
form.second_zip.value = form.zip.value;
}
else{
form.second_address.value = "";
form.second_address2.value = "";
form.second_city.value = "";
form.second_state.value = "";
form.second_zip.value = "";
}
}
//-->
</script>