A lot of people are not aware of the emotional and social level of rabbits, they are very sociable animals and also can create an emotional bond with each other and their owners. For this reason, you should always keep two rabbits together instead of having only one. Rabbits tend to get lonely and depressed when they are kept by themselves.
If one of your rabbits dies, introducing a new rabbit into its habitat can be a very challenging feat. Instead of introducing the new rabbit to the older rabbit in its cage or home, you should pick a neutral ground to have them meet. The older rabbit will act territorial if the new rabbit is placed in its own environment.
One good way to have the rabbits meet is to introduce them in a different room of the house where the older rabbit does not typically go. Place the new rabbit among some small stuffed animals, and let the older rabbit come up and gradually introduce them.
Another idea is to rub almond extract on the rears of both the new rabbit and the older rabbit. Rabbits use the sense of smell to determine their territory, but if you introduce a new neutral odor on both rabbits, they will not feel as threatened by the presence of each other.
Make sure that you are present when introducing the new rabbit, watch the behavior of the older rabbit and if he seems to be taking to the new rabbit, it is ok to let them share the same space, but if you sense that there is likely to be problems, you are going to want to continue this a little bit at a time until you feel that the rabbits are going to be able to get along.
You will know that they are taking to each other when you lay the new rabbit by the older rabbit and they start grooming each other. This is a sign that things are okay for you to put the two together.