I know this is an older post but I just wanted to add in my experience. We currently have two dogs, but started with one about 3 years ago whom we adopted from a rescue at about a year and a half old. He did great as an only dog for the first few years, but we moved to a new place and he had new doggy neighbors - a pair of males who are the bestest of buds. I'm not sure if it was him seeing the two of them play together or what but he started seeming lonely (he'd just sit in the yard and wistfully watch the neighbor boys).
This year we got him a buddy/us a second dog from another rescue who is about about a year and a half. Our older dog seems happier since he has a friend to hang out with and play with.
With that said our boy was very social to begin with - loves loves loves everyone, regardless of whether they're people or dogs or cats. And we waited until we found the right dog for our family. They're great together, but it is a bit more to handle and there's no way I'd want two young dogs at the same time, even two "adolescent" (year-2 year old) dogs. Too much energy and orneriness in my opinion. (As we discovered when we fostered a 6mo old puppy - oh my goodness 3 dogs is one too many for us!).
It's also much more difficult to train two dogs at a time. It's a lot easier when one is already adapted to the ways of the house and what's expected of him/her so you can focus more on the new kid.
Hope that's helpful to someone.