From everything I’ve learned on this site, unspayed/neutered bunnies can never be truly bonded as the hormones and territory issues are just too much. Boy and girl bunny bonds are easier than two boys, but they aren’t impossible.
Did you do any pre-bonding between them? Unfortunately you can just put two bunnies together…. the new bunny Winston might have been more shy and timid and scared while he got used to his new home… then once he was BAM he tried to assert his dominance and Atticus was like “Wait a minute, I was here first, I thought I was the alpha”.
Have they fought enough to draw blood? Bunnies can hold a grudge. You may have to move Winston to another room completely where they can’t see each other and essentially “forget” about each other.
They have Winston neutered – while they are fully separated. He will go through some hormonal surges during this time and be even more all over the place…. BUT the good news is that everything I’ve read is that after their hormones settle they will smell like a completely new rabbit to the other one.
After a month of healing and hormonal balancing out for Winston you can start swapping their cages, litter boxes, toys… so that they start to get used to each others scent in their space…. then start short “dating” sessions in a neutral location = a place neither bunny can claim as their own and slowly extend the time together.
There’s a few more steps after that. But I think that may be a good enough starting point to get this all turned around for you.
There are people on here with more experience than me – so hopefully they will chime in too.