Alas, the best efforts can still be subverted by a random nose to nose.
If not too intrusive, I offer the following personal observation or observations.
It has been my fortunate experience that immersion of a new dog into the ritual of the family, and that includes the family members that are mutts, works for me. Are there occasions when one dog or another takes issue with a new dog? Yup. That's where I come in.
Had a dust up two weeks ago between the two highest drive dogs in the family, and it happened because a piece of carrot got accidentally dropped on the floor. Two dogs went for the scrap at the same time and neither was inclined to back down. Happened in a heartbeat. A treat gets dropped on the floor all the time, well not all the time, and the dogs seem to respect the first come first served protocol Not the other day.
It was two GSDs who were focused on a fight to the exclusion of commands. You got to be a bit nuts to bust up two really angry GSDs. I qualify. They had to contend with a really, really angry fellow.
This all by way of noting that dogs who have been proximate to each other, who get along without attitude for months, can have a bad moment. The solution is to keep them separated for fear a bad moment might occur, or have them together as a lifestyle choice and be prepared to rectify a situation should it occur. I choose the latter.
I also choose to forgive myself for problems that might occur. Put another way, just as my four children infrequently had their moments that required a decisive intervention, so also do I understand that stuff happens with the GSDs that will also require a decisive intervention.
I do not tolerate bad behavior, which I also define. My rules. But stuff happens. I try to learn from it, put it into my things to be aware of folder, and get on with it. You have two really good dogs. They have a really good owner. But in the world of dog interaction, you are a human. You get to set the rules, enforce the rules, modify the rules, and create the atmosphere in which the dogs live. It is up to the dog to follow the rules, and not for you to feel bad when something goes amiss.
Stuff happens, and it may happen again. Life happens. If it happens every day the problem is more you than the dogs. If it happens once a year, it is on the dogs and a full moon.
Under no circumstance should you beat yourself up over a situation. Just get on with it. If you treat both dogs evenly, don't have a favorite, try to accommodate perceived idiosyncrasies, and set clear boundaries, then stuff will work out. That has been my experience.
And it is a human thing to ascribe characteristics more human than dog to dogs. It is also counter productive when setting boundaries for dog interaction.
The good news is that dogs forget quicker than people. Makes for an easy recovery from an incident if the human doesn't intrude too much in the recovery.
Too, too much coffee. Not very useful stuff but I'm trying to master this new laptop keyboard, with little success.
By the bye, the two mutts that went at it last week, are down, resting back to back at my feet. Go figure.
Mike A.
"I wouldn't touch that dog, son. He don't take to pettin." Hondo, played by John Wayne