I love this semi-chapter from one of my favorite rabbit-related books, Living With a House Rabbit, by Linda Dykes and Helen Flack.
The first thing to say is that, unless you are starting off with baby rabbits, both parties must be neutered. The easiset pairing is introducing a new female to a resident male. Most male rabbits are quite happy to welcome a new lady friend into their abode. Female rabbits are usually more territorial than males and you will need to watch out for outbreaks of aggression if you introduce a male into the home of a resident female. However, serious problems are unusual.
Same-sex pairings, such as female/female and particularity male/male, can be very difficult to establish. Unless you have two rabbits you particularly want to live together, you would be well advised to stick to opposite-sex pairings. If you proceed with a same-sex pairing, you must be prepared for a prolonged bonding period. This may extend to months rather than weeks, if the rabbits dislike each other at first meeting.
Litter mates will already be bonded to each other, so trying to form a strong bond with either of them will be futile. Second, they will also reach sexual maturity at the same time. If you have a buck and doe, you will need to plan the buck's neutering surgery immediately after his testicles descend, or else you could end up with a pregnant female overnight.
Two bucks will need to be neutered simultaneously, before they begin to fight. With siblings, same-sex pairings often work out fine, as long as both are neutered or spayed as soon as possible, since the rabbits grew up together and are already bonded.
Let me talk a little bit about age for a second. If you already have a resident rabbit, the age of that rabbit doesn't really matter. You don't need to make it a priority to find a rabbit of the same age. If you have an old rabbit and want a baby, go ahead and introduce them to each other. Chances are, the older rabbit (if it's a doe especially), will be more prone to liking the baby, as her mothering instincts will kick in and she'll care for it.
Introducing an older rabbit to a baby can be just as profitable. Babies are friends with everyone, and the older rabbit will be the newcomer so he or she will be less inclined to dominate over the baby, whereas the baby will just want to snuggle up and be friends!
My next blog will be about where to buy a second rabbit if you already have one. Like stated above, it's much easier to introduce a new female to a resident male, rather than the other way around, though both can end up working out just fine as long as you have patience.