My dogs HATE each other?

I have a female Jack Russell and a female Miniature Schnauzer. They got along the first week just fine then out of nowhere the Mini attacked the Jack. She punctured her throat. She has attached 2 more times since then. We have tried letting them duke it out and it’s brutal. Then we tried muzzling them so that they couldn’t use teeth, which went on for ages and didn’t resolve anything. Help Please.