HI. I have a similar problem with one of my rescue's. His trouble was he was not socialised when he was young and can act aggressive. He never bites but he does at times, bark and if the other dog comes too close he will nip. Fortunatley he is so good on the recall that I can distract him from other dogs by bringing him back to me. I do, however have to walk both my dogs seperately when they are off lead, as part of the problem is the one I have mentioned, protects the other one and his behaviour is worse. I'm not sure if that helps, but you are not on your own. Good luck x