Oh boy... good thing that boy is getting snipped.
You are doing everything good, it's just Tristan who just can't get a hold of his hormones and becomes very stressed out. He needs help and the neuter should do the trick.
Are you getting both neutered or only Tristan? If you can afford it, I would get both done.
Not much to do to prepare for surgery... make sure NOT to fast your rat. His recover will take about 3 days, his cage should be clean and no bedding, fleece or newspaper is best. He can meet up with his buddy again after the second day. But keep in mind that it can take up to 3 months for the aggression to subside.
Good luck!!!
btw, when you do stick your hand in between the boys, be very careful they don't turn around and bite you.