I also have two boxers. Tegan (female) was almost three when we got Owen (male). Tegan was definatley not happy when Owen came home, there wasn't any fighting, but you could tell that she didn't really want to play with him either. The day that we picked Owen up we put him in that car with Tegan and she pushed him onto the floor and would not let him on the seat for the entire 5 hour drive home!
We really tried to stay out of any "discussions" that Tegan and Owen had becuase we felt that they had to determine who was the boss and their own rules. Their "discussions" never got out of hand, and we never had to step in.
I found that we had to make a special effort to ensure that Tegan received as much or more attention than Owen in the beginning becuse we didn't want her to feel left out or abandoned. It also helped to crate Owen periodically while they were together to give Tegan a break.
Owen was crated while we were out of the house, and Tegan was left to roam free and choose if she wanted to spend time with him - which she did, we would always come home and she would be sleeping beside his crate!
Tegan has been a dominant dog from day one, and we heard that males will be more likely to submit to a female than another female would, therefore we chose to get a male, and that's exactly how it has worked out.
We also took Tegan to the breeders place twice to look at the puppies with us. This helped her to be a bit prepared for the new addition. She was excelent with all the puppies, but she definatley picjed Owen as her favorite, which was really neat becuase he was the puppy that my husband I already liked too!
It may be a good idea to bring PSI to meet the two puppies for himself?!!
Being that PSI is still quite young you may not have a problem getting a male or female, depending on his personality.
Having two dogs IMO is better than one. They always have each other should we not be home, and they alsways have a playmate when we go for walks and don't see other dogs. I really enjoy having and training two dogs with such different personalities too, they teach ME someting different/new everyday.
The vet bills and food are double, but that's to be expected.
Good luck with your decision.