[Potter]

You shouting at Justin doesn't help. You being tense when Justin and Quincy is around each other doesn't help either. Quincy can feel it hence think something is wrong. Justin shouting back at you or Quicny will not help either so STOP SHOUTING! Muzzling Quincy might make him feel that everytime Justin is around, you muzzle him which is in his brain equal to something bad. You need to potray Justin as a keeper of good things and when he is around, good things will happen. Why not try you giving Quincy his favourite treats when Justin is around (but not close to him) instead of muzzling him?

You really have to sit down and think of the subtle ways that makes Justin is nice to have around for Quincy. Things like Quincy won't get locked up when Justin is around. Try diverting Quincy's attention with favourite treats or some playing between you and Quincy while Justin is around (again, not close; just around) then slowly work to playing together. Its going to take a lot of work and patience but things would get better in time.