The behaviorist isnt right. Plenty of people have pairs of same sex rabbits, or more! I currently have three bonded males. One had to be removed due to hormones but once hes neutered i have no doubt theyll go back to being a happy trio
First, youll have to find out why they started fighting. Did anything in your place of living change? Have you changed their litter brand? Hay or pellet brands? Got a new litter box(s)? Do you have atleast two litter boxes (one for each)? Did you move their cage position? Change cleaning products in the room they stay in? Some rabbits have hormone spikes, even after being fixed, is it possible one or both of them had a hormone spike for a week or two?
How long did you allow them to fight before officially separating them? The longer you let them fight it out, the less likely it is that they will bond again. How bad did the fighting get? That will also add into how likely it will be for them to bond. If you let them see one another with a barrier between them, what do they do?