I have two dogs, Zelda and Jett, who love to play together and tend to get along quite well. However, when I use commands with Zelda, Jett becomes aggressive toward her. She will growl and bark at Zelda, sometimes even attempting to attack her. Consequently, Zelda becomes anxious and does not always respond to even basic commands in Jett's presence. Why might Jett behave this way and what steps can I take to remedy this behavior?
I agree with mathilda, but I would also see if you could have someone else handling the older dog while you work with the younger one. Maybe the other person can give him treats while you give commands to the younger one so he becomes more comfortable. Or maybe tether him to a spot out of reach and throw treats to him when he's behaving.