I've had pairs of GSDs living together for long periods several times in my life, and it's always been much harder than one dog. A lot of people get a second dog because they want the first dog to have a "friend" or companion to amuse themselves without any effort on the owner's part. It's the same reason a lot of people take their dogs to dog parks and let them loose without supervision. This can be a real problem with young puppies raised together (search for "littermate syndrome") or if the two dogs aren't interested in each other. Are you prepared to put in twice (or more) as much effort to own two dogs if necessary?
Owning my current dog made me realize more and more what lengths people go to make their dogs less interested in interacting with them. If I raised another puppy I would make every effort to keep him as handler focused as possible.