I dont have GSPs, but I've got a pair goldens, and can share my experience of bringing a second dog in as I've done so twice. All of my dogs have (and probably always will be) been females. Very first dog was a very laid back, submissive, and gentle natured. I brought in a second femaile pup that chased her around the house, jumped on her, took her toys, etc. Older dog was wonderful and couldn't have been more tolerant. There was competition at first when hunting them, but they eventually became quite a pair. I LOVED having 2 dogs. A couple of years ago, the older dog passed and I wanted another female puppy. Got one, and I had some major dominance issues with the now older dog. She was used to it her way and there were some pretty heightened moments with the pup over anything from attention to toys to who got to sit closest to dad.
With time, we got over all of those things, but as I started working more with the pup on hunting/training/retreiving, I had some pretty major issues with the older dog on possessiveness over birds. Literally the older dog would sit in the front of the truck and watch the younger dog in the back seat. If the younger dog even looked backward at a dead bird in the back of the truck after a hunting trip, older dog would growl at her!. Same thing in the field, even if younger dog got to a down bird first, older dog would rush in and claim it. It was terrible and a set back for the younger dog and her hunting skill development, as anytime I knocked a bird down, she was afraid to get to close to it for fear of being growled at, even if the older dog wasn't along on that hunt. For some time, I had to hunt them apart to re-develop confidence in the younger one.
Time, training, and patience overcame all that, and this fall/winter I had a pretty darn good team with the two of them. I probably won't ever be without two dogs, but be prepared for some of the variety of issues you might encounter when you introduce that second dog.
Just remember time, training, and patience.