Are they both neutered or spayed? Are they same sex or opposite. It all makes a huge difference. That is the first step. Then you need to wait until they aren't feeling their hormones anymore, maybe 3-4 weeks or more. If you have tried putting them together and they have fought, then it will be harder to bond them in the future. Rabbits have long memories when it comes to fights. If you haven't introduced them, then start with short 15-30 minute bunny dates to see how they get along. Rabbits are very territorial so it's best if you try to bond them in an area that is neutral territory. Try a small room or a play pen, a big new cage. But you will need to be able to get access to either of them in case something goes wrong. Try getting them to meet, play with them both, give them hay to share or treats. If they fight, you will have to go slower. You might try placing both of them in a carrier and taking them for a long car ride as they often will bond in such close neutral quarters. Each day, work with them together for about 15-30 minutes in neutral territory, lengthen if they are getting along. Take them on rides. Use toys. Have a squirt bottle on hand to stop a fight after they are getting used to each other.