If you difference the two WSDL files, the only difference your should see (apart from a comment timestamp) is this location for the binding at the bottom:
I am not familiar with the code that .Net generates, but you should be able to set or otherwise override that value (search for it in the generated code and see) which in combination with a different username/password will allow you to connect to either org.
Based on one of the answers to How to programmatically modify WCF app.config endpoint address setting?, the code may look something like this:
string location = "https://test.salesforce.com/services/Soap/u/34.0";
WSHttpBinding binding = new WSHttpBinding();
EndpointAddress endpoint = new EndpointAddress(new Uri(location));
SforceServiceClient client = new SforceServiceClient(binding, endpoint);