It's a wonderful experience for me but I did it the easy way, I adopted an adult so I knew both of their adult personalities!
Tulley and Desi get along terrifically, my third rescue is a female and they are wonderful together.
We've had many calls for dogs coming into rescue because as they grew they tried to take the unavailable alpha position but I had it easy knowing the personalities of both already. Desi is a perfect gentleman ALL the time, he was so easy to add to my "pack". He was about four when we adopted him. I think it helps that both boys were neutered young (I adopted Tulley young and had him done before 6 months and Desi has indications of having been neutered young also.)
Ashlie
owned by the rescued boxertrio:
Jillian, 3 1/2year old female
Tulley, 2 1/2 year old male
Desi, 5+ year old male