When I got Gryffon, I spent the first year keeping him separated from Keeta, my older dog. Also did the same with another puppy that I was raising for IPO, and had him for about a year. As others have said, spending too much time with another dog can make them really doggy so that they are always wanting to interact with other dogs, and they ignore you.
Keeta and Gryffon would be allowed approximately one hour of supervised outside play a day. In the house, I often left them out loose, but I had a rule about no rough-housing, so they had to be calm, and busy with their own things, like a bone or a kong. With the other puppy I had, he wouldn't stop easily, so there was some crating and baby-gates set up to ensure separation.
I did walk the two dogs together, but again, I expect impeccable leash manners, so not rough housing or playing during walks. I often took the pup out for walks on his own so we could play and interact and do a bit of training during our walks.
For the first year I had Gryff, I took him to work with me, and he stayed in the car, getting walks and play-time during my breaks and lunch. As a matter of fact, I'm still doing this on a daily basis, 9 years later
I'd take my two dog to obedience class, taking two classes, one after another, each class with one dog. Training for IPO, did lots and lots of engagement, one-on-one tug playing. The extra effort paid off. Keeta and Gryff could be out in a huge empty field, chasing each other full tilt involved in the game (Keeta had a crazy energy that scared many dogs, but Gryff just fed on it), and I could call Gryffon and he would instantly break away and come running to me. If going for an off-leash walk on the trails, Gryff would spend the first five minutes pestering me to play tug with a stick he found, and only after I won't engage him did he turn to Keeta and try to engage her in play.
We often have people come to our club, who are not willing to separate their new IPO pup from the other household dogs, and are very discouraged when the pup shows a lack of interest in chasing the rag, playing with the helper, or focusing on the handler. They we hear: "I don't understand why my dog is so remote and low energy here - he runs all day like a maniac with my other dogs, I never thought he would show a lack of drive and energy."
Not a lack of drive or energy, just not interested in anything else but playing with the packmates.