OP, ours are the same as both were best buddies when the young one was small but when the younger dog got older he simply wanted to dominate for whatever reasons. Attention seeking I think or jealousy. Both wanted to fight to the death to become top dog whenever they left alone. The way we deal with them is to cage one while the other is let out vice versa.
When they really fight they are fighting to the death but nobody fight to death when I am around 😆 My family members could not get them off they last time they had a fight, so it was up to me to grab one from the back of their neck (where the loose skin is and grab very tightly) and yanked one out. I usually yanked the younger more energetic one out as he would not stop attacking the older one. It weight around 20-25kg (like a little fat piggy) and with live weight it was heavier. I just yanked it up to nearly 4 feet off the ground then transferred it to the cage.
Our younger one is a local Borneo mongrel which is also known as “meat dog” (people eat them), while the older one is a Tibetan Terrier we rescued from the street.
Both are very loving to us but fight amongst themselves badly especially the jealous mongrel. That mongrel should taste good but luckily we don’t eat dogs otherwise it will be going in the pot.
🙂
edit: all good advices above …