I will offer my opinion but please know that there are different opinions when it comes to bonding methods.
With 2 rabbits that are having difficulty in getting along, it is my opinion that "stress bonding" is useless because it is only temporary. It lulls us into thinking they are getting along but once the stress is removed, they remember that they don't like each other. Stress bonding
may (I'm being generous here)
may help with bonding rabbits that are already inclined to bond. I have yet to see it work for rabbits that are being difficult.
From your description, the 8 weeks side by side was not too helpful since they still tried to go after each other through the bars during that time. Don't get me wrong. That is a method that is often quite helpful in bonding. It just so happens that these two rabbits weren't being cooperative. Not your fault -- just their personalities.
Then when you attempted to get more serious with the bonding, they still insisted on fighting. In a case like this, it might be best to separate them into two different areas where they can completely forget about each other. They would need to be out of sight and out of smell of each other. If they are kept totally apart for several weeks, then maybe - just maybe - they won't remember each other when they are put together again for a bonding session. It may work, it may not. But clearly, the usual side-by-side method didn't work for them.
Separating them for a good length of time and then introducing them anew might give them a last chance to bond.
I'm afraid, though, that there is a possibility that these two are destined not to bond. It does happen.