If you want another pet bunny just because you love rabbits, make sure that you take precautions and know what you are getting into. Introduce your bunnies as young as possible, so they have time to adjust to each other before hitting sexual maturity. Each pairing has their own pros and cons.
BUCK/BUCK: If not neutered, it's almost impossible for adult bucks to be friends, even if they are siblings and grew up together. They will fight constantly and possibly to the death. Each buck feels the need to establish their own space and attract their own females. Competition is very high among the males of the rabbit species.
DOE/DOE: Does can possibly be friends if they aren't spayed. If they are introduced to each other slowly, carefully, and from a young age, they should be friends. They will most likely not fight but may occasionally have some bickers over territory and showing dominance. If they do fight, it will not be to the death, and the worst thing that can happen is that the rabbits will stay away from each other or seek each other out to show dominance to the other. It can be stressful to the rabbits to be in this situation, so spaying one or both of them is always recommended.
BUCK/DOE: Possibly the best combination there is. Boys and girls are as best of friends as possible, almost immediately and forever. However, if you don't get either spayed/neutered, you'll end up with a lot more bunnies then you bargained for in just a few short months.
Each pairing is not guaranteed to be a match. Some rabbits just hate each other forever, whereas others - even bucks - will be friends immediately. Spaying or neutering the rabbit is always the best solution, before getting another, no matter the gender or age.