You know that for one browser session one requester will be cerated.
-->What you can do is hit your environment URL in browser (Not mashup one) it will prompt to login screen there don't login.[here one requester will be created that you can see in requester management].
-->In the same tab you remove the environment URL and give your file path (mashup file) don't hit unless you open remote tracer.
-->Open your environment in some other browser and go to requester management and select the correct requester and do the remote trace.
Hope it helps you.
If you are not able to understand this concept then please revert back to me.