It is natural for the first dog to try and protect is territory but it’s great that you are wanting to find a solution of them being together.
The dogs need to learn that they are together and that you are the boss. The simplest, easiest way to reinforce that is walking them on a leash. Walk them on either side of you, keep the leash short so they can’t go across you and lunge. Walk them as many times in a day and for as long as you can. It will reinforce the learning that they are together and that you are the leader.
Do not separate them in your presence in different rooms. You need to discourage the first dog from attacking the second. As a rule we’d never allow any dog to bring on an attack. Keep a rolled-up newspaper handy and use it on the dog that brings on the attack. They announce it much before they pounce – their body will become tense, there’s low growling and baring of teeth. Talk to the dog at this stage firmly and tell her not to do it. If she pounces use the rolled-up newspaper immediately and separate.
You need to reinforce the behavior that they can’t go at each other. Dogs need discipline and they need to learn to listen to what you want.
It’s not difficult to bring about this change and it does not take more than a couple of days.
Best of luck.
