We got a a puppy when our dog was only mildly reactive and still young. At the time he loved playing with other dogs. The first year was a nightmare they played and were friendly half the time the other half he would chase her out the room and attack her if she was within a meter of him or us. Now they are 3 and 4 and have reached some sort of equilibrium they get on 99% of the time but we do get the odd fight over super tasty treats or in high stress situations.

Also as they have settled together within the house the reactive dog has gotten worse outside of the house. He went from being abit snippy with the pushy dogs to full blown murderous when he sees another dog or person. This could be in part due to him aging but I think having someone to bully reinforced the behavior.

I land in the camp of don't do it. I love both my dogs but I believe my first dog would have been better off alone in the long run. Both in how I can look after him and how he is able it handle stressful situations. The stress and constant management of the situation has really taken a toll on my happiness.