I think I would start with letting them see each other, if that is not already going on. Let one out playing in front of the others cage and then let the other one out for a while with the first rabbit back in his cage. You can see how they respond to seeing their old "friend" again and see if they seem aggressive or even interested at all. If everyone seems to be ok with seeing each other you can try to have some highly supervised interactions between them. Make sure you have an eye on them at all times, and you might even want to get another person so that you can easily separate them if you have to. You also should have lots of toys and hay to distract them if you need to. If these introductions go well then you can think about housing them together. You should take it slow, and it might take a while but it should be possible to reintroduce them together. One thing you should know is that rabbits hump each other even when they are fixed and this is not aggressive just a nervous response, you just have to be very careful if they start that behavior to make sure that neither of the rabbits are being injured, they usually can be distracted into stopping and focus on being petted, or food.