title: Reactive Extensions combining two IObservables into one
body: <p>I'm trying to combine an IObservable that contains request information with an IObservable that contains response information.  Both the request and response information contain an Id and a timestamp in the form of a DateTime.  The Id on the Request will match the Id on the Response.  </p>
  
  <p>I need to combine the two streams based on Id and calculate the time elapsed between the Request's timestamp and the Response's timestamp.  What is the best way to accomplish this:</p>