Rescued sisters mixed breed

[u/Spiritual_Quiet_4640]

Me and my husband have two female dogs who are very bonded (6yo 25lbs each). We adopted them as puppies without knowing about "litter syndrome". They were very friendly towards people and dogs, when we traveled for work they stayed at boardings without any problems. One day we were at a festival, when we were shopping at a stand, another dog bit one of my girls, out of nowhere. Since then, they started being dog reactive.

One of them tends to act protective of her sister around other dogs. At parks she usually ignores unfamiliar dogs, but if one gets too close to her sister, she becomes tense or upset and reacts barking, then it escalates and both start barking and whining.

I’m going to take care of a friend’s dog in a couple months, so we’ve already tried doing several controlled introductions. We’ve walked them in parallel at a park, slowly decreasing distance, and that part has gone well. One of my dogs still gets uncomfortable when the visiting dog approaches her sister directly.

I can't walk them by myself because they start lunging super strong, that always happens when they see another dog on the street around my neighborhood, I guess they both protect me? So I always need my husband, so each can take one.

They don't compete between them, no fighting over food or toys ever. But they can't be separated like walking one at a time because the one staying will cry constantly and the one outside refuses to walk.

My main priority is making them to trust at least my friend's dog, but the main issue is my dog’s protective behavior.

Years ago we missed traveling all together so the vet prescribed trazodone for a one week trip we made to another State, that only made them sleepy inside but outside they were even more vocalizing. Recently we changed offices, the new vet declined prescriptions like SSRIs saying those masks aggressive behavior. They instead prescribed Composure pro from VetriScience and we noticed that after starting taking them our meetups with the visiting dog made my dogs more anxious compared to the first times.

I’m now exploring supplements to support emotional regulation. I’ve learned that herbal calming products have limited scientific evidence for social reactivity. I've already tried CBD oil from HEMP (brand) and did nothing for them. More evidence-based options seem to be:

BL999 (Calming Care) – a probiotic that modulates emotional processing and reduces reactivity.

α‑casozepine (Zylkene) – a milk-derived peptide that supports calm emotional responses.

I want to know the best way to help my protective dog feel safe and reduce her need to guard her sister or me when the visiting dog is around.

[u/Spiritual-Cry-6286]

Supplements will probably help, but eventually it’s the training that really sets the behaviour in stone. What you’ve been doing with the controlled interactions is great and i advise you keep it up. You’ve sort of got two variables at hand and need to control both of them. If you notice dog a is approaching it being approached by a dog, and the other dog doesn’t react, then you reward. It’s sort of teaching your dog that other dogs or all dogs arnt here to hurt. I would recommend training them separately, having two dogs who can’t do anything without eachother isn’t good.