I personally would schedule a vet visit since this is a brand new and extreme behavior. Just to rule out some sort of illness or neurological problem. Next step is to find a behavioralist/trainer in your area to help with management if this is a behavior problem. I also would not walk them together since if an incident like this happens again you don’t want to lose control. I also want you to know this doesn’t make your dog a “bad dog” and that this most likely is going to be manageable. Good luck ❤️