Can't call interfaces implicitly from only dll

I have class as  public static class ConsumerRequestExtensions , this class has method as a
  public static IConsumerRequest Get(this IConsumerRequest request),
i want to use that Get method , so i created object of IConsumerRequest and call that Get method by the IConsumerRequest object, but it gives error that explicit conversion exists.
because i am using two Dll's which have same interface, so that i create alias for one dll and access IConsumerRequest interface , but implicitly IConsumerRequest not calling the Get method of ConsumerRequestExtension.

code is here.

 OnlineQBO::DevDefined.OAuth.Consumer.IConsumerRequest conReq = oSession.Request();
                conReq = conReq.Get();

My dlls are
XeroApi.dll
DevDefined.OAuth.dll

please suggest any solution to avoid explicit calls of dlls

Thank you..