Chat system doesn't update automatically online

I am currently working on a chat system for a school project and I have a problem with updating the chat while 2 users talk simultaneously. I am using ajax, javascript, aspx, html, c# and xml. I save every chat between two users in an xml file. Everything worked great, until I checked if the chat update automatically when both users talk online. To check that, I opened the project with 2 microsoft visual studio programmes, logged in with 2 different users and try to send messages. The xml file has been updated in both visual studio programmes but the chat only updated in one tab. I only see the new message if I refresh the page.

How can I update both tabs automatically without refreshing the page?

Here is the code that loads the conversation for the first time:

<div id="Conversation">
                <script type="text/javascript">
                    /*Load xml Document */
                    var xmlDoc = LoadXMLDoc(MakeFileName());
                    /*Checkes which users are talking*/
                    from = document.getElementById("ContentPlaceHolder1_SenderID").value;
                    to = document.getElementById("ContentPlaceHolder1_ReciverID").value;
                    /*Get all messages*/
                    message = xmlDoc.getElementsByTagName("text");
                    for (i = 0; i < message.length; i++) {
                        /*Create new p element with the message's text*/
                        var PElement = document.createElement("p");
                        var TextNode = document.createTextNode(message[i].childNodes[0].nodeValue);
                        var DivElement = document.getElementById("Conversation");
                        /*Write the text in the right direction according to who sent the message*/
                        if (from == xmlDoc.getElementsByTagName("from")[i].getAttribute("id") && to == xmlDoc.getElementsByTagName("to")[i].getAttribute("id")) {
                            PElement.appendChild(TextNode);
                            DivElement.appendChild(PElement).style.textAlign = "right";
                        }
                        else
                            if (to == xmlDoc.getElementsByTagName("from")[i].getAttribute("id") && from == xmlDoc.getElementsByTagName("to")[i].getAttribute("id")) {
                                PElement.appendChild(TextNode);
                                DivElement.appendChild(PElement).style.textAlign = "left";
                            }
                    }
                </script>
            </div>

Javascript file:

var xmlhttp = LoadXMLHttp();
var xmlDoc = LoadXMLDoc(MakeFileName());
//crate an xmlhttp request that support all web browsers
function LoadXMLHttp() {
    var xmlHttp;
    if (window.XMLHttpRequest)
        xmlHttp = new XMLHttpRequest();
    else
        xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
    return xmlHttp;
}
// load the xml document
function LoadXMLDoc(FileName) {
    url = "Conversations/" + FileName;
    xmlhttp.open("GET", url, false);
    xmlhttp.send(null);
    return xmlhttp.responseXML;
}
//create the file name to retrive xml document
function MakeFileName() {
/..../
}
//create a new xml element and adds it the the xml document
function CreateXmlMessage() {
/..../
}
//prints the new massage on screen
function PringMessage() {
    var PElement = document.createElement("p");
    var TextNode = document.createTextNode(document.getElementById("MessageText").value);
    var DivElement = document.getElementById("Conversation");
    PElement.appendChild(TextNode);
    DivElement.appendChild(PElement).style.textAlign = "right";
}
function SendMessage() {
    if (document.getElementById("MessageText").value != "") {
        CreateXmlMessage();
        //create a post request to Chats.aspx
        xmlhttp.open("POST", "Chats.aspx", true);
        xmlhttp.setRequestHeader("Content-Type", "text/xml");
        xmlhttp.setRequestHeader("IsMessageSent", "true");
        //Parse the xml document to string
        var XMLString = new XMLSerializer();
        var sendStr = XMLString.serializeToString(xmlDoc);
        //sends the string to the server
        xmlhttp.send(sendStr);
    }
}