To further drill down on your original question, you need to set up a test case that calls each of the two services.
The response (or desired content) from each service can be "filtered" into a DevTest property. The Filter is applied in the step where the WS call is made.
To simplify the model, a third step (Do Nothing) can be added which contains the Graphical Side-by-Side Assertion. The inputs to the Assertion will be the property names that you filtered in your webservice calls. For sake of example, the fl_WSResponse1 property might be filtered in the step WS Call to Env 1 and the fl_WSResponse2 property might be filtered in the WS Call to Env 2 step. The Assertion uses property notation {{ }} to reference the two XMLs.
Use the Settings tab to configure the desired behavior with regard to how the comparison should work for your specific implementation.