I have always let my buns date and pick out their new mate. When Cinderella died Houdini went into a deep depression. I took him on dates. The first contender was named Pepper. He nipped at her. Then we tried Sophie, Pepper's sister. They looked identical other than color, but Houdini could tell a difference. As soon as Sophie came into the room he started grooming himself, which he had not done since Cinderella's passing. He inched forward and sat smushed up against Sophie for several minutes before they started to sniff around the room together. They have been together ever since.
Bonding would have been much more difficult, if not impossible, if I had just brought Pepper home never knowing his preference for Sophie. There must have been some sort of subtle bun communication difference.
These two might just rub eachother the wrong way and never get along. If they are hurting each other then you might want to give up. If it is light nipping then try stressful situations (like a car ride or in a basket on a running clothes dryer).
