We had a 2 year old male staffy and got a 2 year old bull terrier (breeder had kept her to show, then sold her as an adult). They were born only a couple of weeks apart and we had had staff since 8 weeks old.
We started very carefully with a baby gate and one dog in the kitchen, one in the living room. And introduced them by walking them together. Anything could have happened. Our staff was good with other dogs, but we had no guarantee that a 2 year old dog who had spent most of her life cooped up in a kennel in a garage would be OK.
Within days it was obvious they could live together just fine. They spent the whole time stood at the baby gate, admiring eachother. So the baby gate came down and they lived together very happily - never a spat - til 4 years later he got ill and died. Have, to say that was the hardest part of having 2 dogs - when he died so suddenly, she was left absolutely heartbroken and bereft. She shook all day barely ate - it went on for weeks and weeks. Slowly she returned to something close to normal but she was never the same. We decided not to get another dog - just give her all our attention.
She went blind when she was 4, and that was a bit hard having two as well, as we had to walk them separately (they played too boisterously and it could have damaged her eyes further).
We were just saying today we don't know how we'd have managed had he lived to 14, like she did. As she had dementia the past couple of years and also went deaf the last year. If we'd had two maybe with different health problems, that would have been tough.
Not being negative though - as they loved eachother so much, and got so much happiness from being a pair. If it works that well though, it is tough on one dog when the other dies.
We had a cat, too. She was fine with the 2nd dog but always loved the 1st dog a lot more! Didn't hide her favouritism.