Can a web service client with identical signature call a different web service?

My project has gotten into web services lately, but our QA team has a request to be able to set up a clone client and clone web service so that they can issue a request from the clone client to the real web service and from the real client to the clone service to test the real pieces individually.

The clone client to real web service part is easy, I just wrap the generated client in a main() app that picks up data to send that the test team specifies. But the service part is confusing to me. How can I make a dummy service that just echoes out its requests to a log without affecting the real service? I want the real client to use the same generated client code, just point the soft-coded URL to the URL of the dummy.

If I define a new web service with identical names and signatures to the real service, can the real client hit the dummy with just a URL change? Or is it more complicated? Am I barking up the wrong tree?