Rabbits are extremely territorial. In wild rabbits, territorial behavior includes depositing marking pellets at the boundaries of the territory, chinning, urinating, and aggressive behavior such as digging, circling, and fighting. In our neutered domestic companions, hormonal causes may be absent, but territorial behavior still exists. Thus, when introducing new rabbits, territory must be considered and used to your advantage.
Always introduce rabbits, regardless of sex or age, in neutral space first. Try to eliminate the possibility of any territorial behavior by choosing introductory spaces that are as different from your bunny's territory as possible. Make the space small enough so you can control the interactions, and make sure there are no small spaces a bunny could get trapped in if being attacked.
Possible neutral spaces might include:
If the bunnies show any signs of aggression, try:
The noise and movement will be slightly frightening to the bunnies and they may snuggle up and draw comfort from each other creating positive memories of each other. They will associate the other rabbit with a sense of security as opposed to carrying bad memories around with them if their introductions result in a fight.