Neutral territory is imperative. Beginning the bonding in a small, neutral territory is vital to this relationship working out. DO NOT put them in a cage together until they have been getting along perfectly for several days. If a serious fight breaks out in a cage, it is generally difficult to separate them. Although these behaviors may look similar to you, they are all very different and it is important to recognize the different ways rabbits speak.
Fighting is usually an instantly, purposely vicious attack.
Rabbits sometimes attack the other rabbit's face, underside or genital area. DO NOT separate the rabbits unless they are truly fighting. Every time you separate the rabbits, you will have to re-introduce them, making this process take even longer. Nipping at each other in a playful manner is not usually serious and is done for several reasons. This too should dissipate as they learn how to approach and communicate with their new partner.