Blunt but if you unable to get any behavioral training, this is really the only alternative.Keep the two separated 24/7.
I'm in Minneapolis dog-sitting for my son's two dogs. It was going to be for ten days but he's stranded overseas until at least April 16.
I can't stay here that long so I'm packing up the dogs and driving them five hours to my home. The problem is that one of them, the big, young cattle dog mix, has already demonstrated that he can't be trusted with other dogs. He barely tolerates the one he lives with.
So I'll keep him separated. I've had some experience getting dog-aggresive dogs to tolerate specific dogs, so I'll work on that, but it's an arduous process and I'll never leave him alone with my dogs and I'll never be able to teach him to generalize and tolerate ALL dogs.
Having multiple dogs in a household that can't be safely left together is a huge PIA for a month or two. In the long run, it's incredibly sad.