We had the same problem with our dogs. It didn't help that both were females
We had the cattle dog first, then got the mastiff. The cattle dog claimed owner ship from day 1. She nearly did kill the mastiff one morning.
We did have the cattle dog tied up for a bit, then we tied her up only after a fight. In the end we just had to watch them constantly.
Eventually they got better. The mastiff got bigger, but was always a sooky baby, so the cattle dog still bullied her.
They were both great with the kids.
All I can say is maybe just wait it out & watch them. Our cattle dog decided chasing trucks was more exciting & got lost, so that ended the fighting for us.
Sorry I don't have any advice