Overall, it seems that all you need to do is cast the result of request.getFirstService to SecondService. If your unmarshalling works, and the result of your getFirstService() is not null, then for sure you're getting a concrete type.
If you're still stuck, you have to provide additional information, which should include a minimal set of WSDL, XSDs and sample request, to reproduce your problem, the version of your JAXB provider, and the configuration you've used to generate your code.