Async Http Handler

Question

Looking for the correct way to run multiple api request from a single async http handler. The sample code below works for executing one process but I will like to run multiple at the same time (i.e. ar1.ProcessRequest, ar2.ProcessRequest, ect.). What would be the correct, and most efficient, way of achieving this?

Ex:

class AsyncHandler: IHttpAsyncHandler

{

   public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object state)

   {

APIAsyncResult _ar

_ar = new APIAsyncResult(cb, state);

_ar.ProcessRequest(_context); return _ar;

   }

    public void EndProcessRequest(IAsyncResult result)

    {

        _context.Response.ContentType = "text/plain";

    }

    public void ProcessRequest (HttpContext context) 

    {        

    }

    public bool IsReusable { get { return false; } }

}

//async class

class APIAsyncResult : IAsyncResult

{

    private AsyncCallback _cb;

    private object _state;

    private ManualResetEvent _event;

    private bool _completed = false;

    private object _lock = new object();

    public APIAsyncResult(AsyncCallback cb, object state)

    {

        _cb = cb;

        _state = state;

    }    

    public Object AsyncState { get { return _state; } }

    public bool CompletedSynchronously { get { return false; } }

    public bool IsCompleted { get { return _completed; } }

    public WaitHandle AsyncWaitHandle

    {

        get

        {

            lock (_lock)

            {

                if (_event == null)

                    _event = new ManualResetEvent(IsCompleted);

                return _event;

            }

        }

    }

    public void CompleteCall()

    {

        lock (_lock)

        {

            _completed = true;

            if (_event != null) _event.Set();

        }

        if (_cb != null) _cb(this);

    }

    public void ProcessRequest(HttpContext context)

    {

//api call stuff here

    }

}