asp.net call WebMethod from Javascript asyncronous

I am trying to build an asp.net(c#) page that updates some state texts every second. Now I have implemented an button that calls another PageMethod which restarts something and takes a little while. The problem is, that when I call the restart PageMethod , the update PageMethod can't update as long as the restart method is proceeding...

I wrote a little example to show what I mean:

WebMethods in my Page:

    [WebMethod]
    public static string Update()
    {
        //return "a" to see when the Update PageMethod succeeded
        return "a";
    }

    [WebMethod]
    public static string Restart()
    {
        //the restart will take a while
        Thread.Sleep(2000);
        //return "a" to see when the Restart PageMethod succeeded
        return "a";
    }

the html elements to update:

<p id="update" style="float:left;"></p>
<p id="restart" style="float:right;"></p>

the Pagemethod calls:

callUpdate()
            function callUpdate() {
                PageMethods.Update(function (text) {
                    //itself+text from pagemethod
                    $('#update').text($('#update').text() + text);
                });
                setTimeout(callUpdate, 1000);
            }

            callRestart()
            function callRestart() {
                PageMethods.Restart(function (text) {
                    //itself+text from pagemethod
                    $('#restart').text($('#restart').text() + text);
                });

                setTimeout(callRestart, 1000);
            }

Note: The Update is also called every second after it finished, just to see how it works

To clarify: I want the PageMethods to execute independent to that the other PageMethod has finished.

I also flew over some links like: http://esskar.wordpress.com/2009/06/30/implementing-iasyncresult-aka-namedpipeclientstream-beginconnect/

http://msdn.microsoft.com/en-us/library/aa480516.aspx

But I don't think this is what I need (?) And I really don't know how to call that from Javascript (BeginXXX and Endxxx)

*EDIT: *

Regarding to Massimiliano Peluso, the js code would look like this:

        callUpdate()
        function callUpdate() {
            $.ajax({
                type: "POST",
                url: "ServicePage.aspx/Update",
                data: "{}",
                contentType:
                "application/json; charset=utf-8",
                dataType: "json",
                success: function (msg) {
                    $('#update').text($('#update').text() + msg.d);
                }
            });
            setTimeout(callUpdate, 1000);
        }

        callRestart()
        function callRestart() {
            $.ajax({
                type: "POST",
                url: "ServicePage.aspx/Restart",
                data: "{}",
                contentType:
                "application/json; charset=utf-8",
                dataType: "json",
                success: function (msg) {
                    $('#restart').text($('#restart').text() + msg.d);
                }
            });
            setTimeout(callRestart, 1000);
        }

Note: when I run the Page with the new js, there is exactly the same problem as before: The Update method can do nothing until the Restart method is finished.