We added another boxer, Dino, when Rocco was 4 and Dino was almost 1. That was almost 6 months ago. I guess I'd say it can go either way. They love to play with each other, but they are also very jealous. When I sit on the couch, they both try to get in my lap. Or if Rocco is laying in my lap, Dino stands there and cries and barks. That part drives me up a wall, even though I love having them sit with me. Rocco will sometimes give a warning growl when Dino tries to barge in on us. There's also jealousy over toys. Sometimes they play tug of war together with toys, but then other times Rocco will growl if Dino gets too close to him when he has a toy. They eat side by side with no problem. Rocco, the older one is more jealous than Dino. He has gotten grumpier in his "old" age.