How long have they been separated? Perhaps whacked out hormones (from being neutered) were bottled up and finally resulted in the fighting. Is one picking fights more often than the other?

My two are the same. They would viciously fight given the chance. I found the photo journal in the bonding area to be helpful in helping to read their body language. Also I found putting them in a small pen but separated by a section of the cage. The area needs to be small enough so they can’t really move much so they are forced to be in the vicinity of the other one. Mine fought through the cage – no damage but after a few arguments they settled down and touched noses through the bars. I did this every evening for about an hour a week before moving onto supervised time together in a slightly bigger area. I often have to calm nipping down but the fights have stopped. I’m still along way from complete bonding but with two males I think it can take a lot longer and require much more patience. Keep going with one small step at a time.