ASP.NET Asynchronous Calls

I have an ASP.NET page that needs to make calls to multiple web services to return data. I would like to run these requests in parallel, and using the PageAsyncTask class seems to be the most appropriate way to do this. The trick is that there are five calls — say A, B, C, D, and E. Calls A and B must be made in sequence, just like C and D. Pair AB, pair CD and E can work in parallel.

It doesn't look like I can use the "executeInParallel" parameter of the PageAsyncTask constructor to create 5 tasks and get this parallel / sequential configuration.

I tried to create 3 tasks, two of which have a chain of asynchronous calls (similar to http://msdn.microsoft.com/en-us/library/dwba7yy7(VS.80).aspx ). When I try to do this in the PageAsyncTask context, I get an error indicating that the final function can only be called once. I tried wrapping a chain of 3 functions in a couple of functions that the PageAsyncTask page uses and this results in the final function never being called (thus time-out).

Is there a good way to do this?

Edit: If it's helpful, here's a simplified version of the code I'm trying to get and that is an error that a final function can only throw once.

     Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim objMB As New BusObject.AsyncTasks(AddressOf dummy)
        Dim objTask As New PageAsyncTask(AddressOf objMB.BeginStartSession, AddressOf objMB.EndGetList, Nothing, Nothing, True)
        RegisterAsyncTask(objTask)

        Dim i As Integer = 0
    End Sub

Public Class BusObject
    Public Class AsyncState
        Public SecondCallback As AsyncCallback
    End Class

    Public Class AsyncTasks
        Dim fn_Callback As PageCallback

        Public Delegate Sub PageCallback(ByVal objData As Object)

        Public Sub New(ByVal fnCallback As PageCallback)
            fn_Callback = fnCallback
        End Sub

        Public Function BeginStartSession(ByVal sender As Object, ByVal e As EventArgs, ByVal cb As AsyncCallback, ByVal extraData As Object) As IAsyncResult
            Dim objWS As New com.WebService
            Dim objX As New AsyncState
            objX.SecondCallback = cb
            Return objWS.BeginWSFunction1("param1", "param2", AddressOf EndStartSession, objX)
        End Function

        Public Sub EndStartSession(ByVal objAsyncResult As IAsyncResult)
            Dim objWS As New com.WebService
            Dim strSessionKey As String = objWS.EndWSFunction1(objAsyncResult)
            Dim objState As AsyncState = objAsyncResult.AsyncState

            objWS.BeginWSFunction2("p1", "p2", "p3", "p4", "p5", strSessionKey, True, objState.SecondCallback, objState)

        End Sub

        Public Sub EndGetList(ByVal objState As IAsyncResult)
            Dim objWS As New com.WebService
            Dim objResult As com.WebService.Result = objWS.EndWSFunction2(objState)
            Dim lstReturn As New List(Of BusObject)
            fn_Callback(lstReturn)
        End Sub
    End Class
End Class