A good trainer should be open to other trainer's opinions and be able to explain why they agree or disagree with this opinion or exercise in a constructive manner. You may be surprised to find that your trainer is excited to learn about the new things your current trainer has taught you. Your trainer's job is to teach you, so if you're confused by something OR discovered an exercise or technique which really helped you improve, you should share that with your old trainer so that they can help you build on your current success. Often you will find that your old trainer agrees with your new trainer (or vice versa) and that they have simply chosen to focus on different things or approach a problem with 2 different solutions, both of which could be considered correct---one just works better for you or your horse. If they are offended, then perhaps you should consider finding a new trainer when you're home.