function redirect() {
    if (document.fred.ID[0].checked) {
        document.fred.action = "doneN.pl";
        return true;
    }
    else if (document.fred.ID[1].checked) {
        document.fred.action = "doneP.pl";
        return true;
    }
    return false;
}

##</code><code>##

<form name="fred" onSubmit="redirect()">