I have kept mine separately because of everything I read, and have been told by more experienced keepers.
They can see each other, and nose through the wire. This past year when I had my accidental litter, I separated mom out for a few days to wean them. I tried putting her with her kits after that and she started riding them around and biting the back of their necks. I quickly separated her out again.
My males will attempt to bite each other through the fence, and they would fight to the death I believe.
I think it's a case by case basis, and depends mostly on the individual rabbits involved.