He is extremely pair-bonded to the mini, and in his mind you are not important AT ALL. It is dangerous because you are not really in control of his actions, and it is not good for any of you because you will never have a safe ride while his mind is more interested in his friend than you.
I understand that you have tried to reduce the bonding by separating them, and that is hasn't worked. Well - you must try more, and succeed.
There are many here that can give you good advice on training issues, so I will only suggest a couple of things:
1 put the mini away out of sight when you are working the TB. Always.
2 lunge him on a lunge line, not free. If you are not proficient at this, then get a trainer to help you.
3 if any of this is on the edge of your skills, or out of them, then get a trainer involved. Otherwise this is going to get worse and someone will get hurt. You will also have a TB that is no good to anyone, and that is not fair on him.
It CAN be done. You can cut this bond. And it really is imperative.
Get up, get going, seize the day. Enjoy the sunshine, the rain, cloudy days, snowstorms, and thunder. Getting on your horse is always worth the effort.