Errore Js fare più richieste con ajax

Ciao non sto riuscendo a fare 2 richieste usando l'ajax contemporaneamente perché...

Codice PHP:

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function HttpAjax(handler){
var xmlhttp = null;
try{
//Firefox, Opera e Safari
xmlhttp=new XMLHttpRequest();
}
catch(exc){
//Internet Explorer
try{
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
catch(exc){
try{
xmlhttp=new ActiveXObject("Msxml2.XMLHTTP");
}
catch(exc){
alert("Il tuo Browser non supporta AJAX.");
return false;
}}}
xmlhttp.onreadystatechange = handler;
return xmlhttp;
}

function Discussione(){
if(Ajax.readyState == 4 && Ajax.status == 200){
document.getElementById("discussione").innerHTML = Ajax.responseText;
}}

function Online(){
if(Ajax.readyState == 4 && Ajax.status == 200){
document.getElementById("online").innerHTML = Ajax.responseText;
}}

function Ricevi(){
Ajax = HttpAjax(Discussione);
Ajax.open("GET","message.php");
Ajax.send(null);
}

function Ricevi2(){
Ajax = HttpAjax(Online);
Ajax.open("GET","online.php");
Ajax.send(null);
}

window.setInterval("Ricevi()", 1000);
window.setInterval("Ricevi2()", 100);


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