这是我写的一段ajax的代码片段。
createXMLHttpRequest();
        url="valindex?username="+document.myform.usern.value+"&userpassword="+document.myform.userp.value;
        xmlhttp.open("post",url,true);
        xmlhttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
        xmlhttp.onreadystatechange=callbackpassagain;
        xmlhttp.send(？？);
我想传的是两个变量。分别是绿色标记的username和userpassword。这里面标记为红色的那段代码的括号里我想传两个变量该怎么写？请指教。

createXMLHttpRequest();
value1=document.myform.usern.value;
value2=document.myform.userp.value;
        url="valindex?";
        xmlhttp.open("post",url,true);
        xmlhttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
        xmlhttp.onreadystatechange=callbackpassagain;
        xmlhttp.send("username="+value1+"&userpassword="+value2);