Pepper had only just turned two when we got Kane (puppy) so she was very much still in the crazy energy, play all the time, hyper phase of her life. She was pretty thrilled to have a playmate 24/7 and couldn't get enough of the play. Kane like to play too, but tired out more easily. After the first few days Pepper realized this little puppy was not leaving and the novelty wore off a bit. Mostly when it came to sharing. Pepper was used to being the only dog so even I didn't realize she had a bit of resource guarding when it came to her toys. Also they both RG their food. The problems really started when Pepper would try to correct Kane (for taking her toy away, or trying to rest his head on her while she was sleeping, or any other normal dog correction). Kane has issues we also didn't know about. He tends to mirror the other dog's actions without really thinking about it himself. So when Pepper would get irritated and snarl or snap at Kane, instead of learning from that Kane would get angry too, but not back down. And so the "scuffles" started. It was pretty scary to me and I didn't really know how to handle it. I got quite upset when it would happen. I eventually realized that I needed to control my own emotions in those moments because the dogs could sense that I was upset and stressed and that wasn't helping them at all. They still played and wrestled together like crazy, but these "episodes" would happen frequently.
It took time, but we've mostly learned what will trigger an "episode" between them and we avoid those situations. They get fed separately, when they get new toys they have to be introduced in the right way at the right time and we (humans) can't get too excited or make a big deal about it, if they're getting treats they can't be sitting right beside each other, etc. Kane is fearful too so if I'm walking them and we have to pass another person or dog, Kane will kind of freak out. Then when he can't get to the person/dog, he will turn on Pepper. I don't think he even realizes it's her in that moment of fear and panic. It's sad really; I feel so bad for Pepper sometimes. But again it's something I've learned to manage by avoiding the triggers.
Today Pepper is almost five and Kane just turned three. They are best friends and still love to play and wrestle. Pepper has slowed down slightly (finally) so it's not as intense as it used to be. Pepper doesn't really defend herself much anymore though. If she can see Kane is going to be upset about something she just backs right off and tries to avoid the situation herself. Kane has also grown up a little and has gained some confidence, though he still requires management.
So I'd say about 95% of the time they get along fabulously and are great friends. If we go hiking or something, Pepper is wandering, running, sniffing, etc. and Kane is following her around doing the same thing. If Pepper leaves the area, Kane leaves the area and follows her. As far as "episodes" go, they happen maybe once every few months, so a few times a year they will still scuffle. Most of the time I can see I messed up somehow, but every once in a while Kane will freak out on Pepper and I'm completely baffled as to why. But most of the time everything is fine. They've never actually hurt each other, it's mostly noise and showing of teeth and it doesn't last long (30 seconds at most).
Our dogs don't cuddle or anything. Kane would because he's actually quite affectionate, but Pepper doesn't like it.
I wouldn't have known any of this from the dogs meeting before we decided to get Kane, but I will never get another dog where I can't meet the parents. I think Kane's issues are mostly genetic.
The consequences of having multiple dogs that can never be together are just inconceivable to me, though people do it.
I would never want to be in this situation either. For a while I thought I was and it was devastating and incredible stressful. Fortunately we have been able to work through their issues and have found a balance.