Sadly I think you're right to separate now if you can't be with them tomorrow. I would be inclined to try again in a while when you have several days to stay with them, use a small pen (around 4*2 or maybe even smaller to start off with if they are skittish and that is winding them up) in a neutral area.
What I find works well to start off with is to hover over them, and to separate them before fighting rather than taking a broom to them once they have started. So for instance if one heads towards the other with ears forward, tail down looking like they want to sniff and say 'hello', if the other one looks as if they are going to respond aggressively (ears back, tail up etc), I will gently put my hand on their back. It seems to calm them down and they allow the other bun to sniff them and back off without a fight. If they do lunge, your hand is already there to stop it from escalating. I find that this keeps things much calmer as they learn much more quickly that they can approach each other without being attacked. Over the course of a few hours you then normally find that you can take your hand back, if they see your hand there they know that they can't lunge, so don't. And then after a few more hours all it takes is a quick shout of "oy" and they will back off if they are starting to look aggressive.
I bonded my boss' rabbits where they had said the female was so aggressive that she just went for him every time he went towards her for a sniff. By keeping a firm hand on her back for the first couple of hours, she got all submissive and by the afternoon she was binkying in the run, so it can change massively quickly if you can help to calm down their anger or fear. Many buns attack as a defence mechanism (i.e. you might attack me so I'm going to attack you first) and this helps them to understand that the other bun isn't going to attack them, they're just coming to say hello.
Although given your current supervision times I agree with you having separated them, the problem with taking them out to let them 'calm down' during bonding is that it doesn't help in the long run. They still need to sort out their hierarchy so it will just continue next time you try. I find the trick is in knowing the body language and stopping them before they fight rather than separating them after. Then you see the body language change over the course of the day as they settle.
I do think there is hope for this bond; they obviously don't inherently hate each other otherwise you wouldn't have had some of the earlier success that you did, so it sounds to me like it's just a case of managing their interaction slightly differently to prevent anything from getting to the stage where it needs separating an active fight.
Good luck x