You are the pack leader, when the older dog starts to dominate or take charge YOU stop her to show her it's not acceptable Watch the dogs body language and movements. You do not need to talk or yell, just be calm but assertive, stand in between and make your older dog give up first. I have 3 dogs and any other dog that comes to my house is welcomed and accepted by me, my pack also accepts. Do not play favorites, Now that you have two dogs you will see it's less of a human understanding but more of trying to understand the 2 dogs as you pick up on their signs on how they deal with dominance, they will always try to compete and take charge if they know you don't have 100% control. You might have to take away all the toys.balls etc, this will also get them fighting for dominance, and even though you may buy 2 of something, the dog like child will fight for what the other has. But make sure that the puppy also respects the older dogs space.
Good Luck