I think it sometimes depends on the dog, and their current mood. Sometimes one of mine will start barking, and the other will join in. And sometimes the other one won't. One thing is for sure for my situation is that adding a second beagle did not alter the amount of barking in he first. Actually, I think Ruby is more protective of outside movement and noises now than before Bailey came along. But all in all, they are both not heavy barkers - just a little protective sometimes.