As the owner of two dogs, I think that two is a wonderful number! However, I also think that your current dog deserves all your time and attention in order to navigate the ups and downs of doggy adolescence.
Once your current dog has been able to benefit from all that hard work you put into training and socializing (and at just 7 months, your current dog has a whole lot of learning and maturing to do still), then it would be a great time to look into bringing a second dog into your home. For most German Shepherds, that is about 2-3 years in age.
If your puppy has a lot of puppy energy, up your exercise and training regimen. Make your current dog as good as he can be! And then consider another dog.