I assume you're creating this in DevTest Workstation.
If you add the Request Data Manager DPH, it provides a UI to copy pieces of the request from one place to another. You would select:
Copy; (From) Meta-data; X-B5-TraceId; (To) Argument
Request Data Manager Data Protocol - DevTest Solutions - 10.3 - CA Technologies Documentation
and you will see that your X-B5-TraceId is now an argument that is used for selecting responses.
If you use "Keep" in Request Data Manager, it removes all the other arguments. You say that all other details in a request are the same, so you might want to look at this as an extra option.
Rick