Cannot call multiple web service methods using ASP.NET AJAX at the same time

I really can't figure this one out... I've combed through a lot of documentation and Google results, too. I have a pretty-standard WCF service, with a contract something like this:
[ServiceContract(Namespace="http://www.domenicdenicola.com/")]
public interface IMyService
{
[OperationContract]
[WebInvoke]
void PerformBigOperation();

[OperationContract]
[WebInvoke]
int GetProgress();
}

I generate the ASP.NET AJAX proxy class in the usual way:
<asp:ScriptManager ID="ScriptManager" runat="server">
<Services>
<asp:ServiceReference Path="MyService.svc" />
</Services>
</asp:ScriptManager>

I then call them from JavaScript like so:
var service = new www.domenicdenicola.com.IMyService();

function UpdateProgress()
{
service.GetProgress(function onSuccess(result)
{
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