Even if they are fixed sometimes two males just can't be in the same cage, so if you see heavy aggressive fighting separate them.
Get something that smells like the new bunny and let your bunny smell it. The first day do that and let the new bunny settle down. On the second day put them in separate cages side by side, do this for about two days or when they both seem comfortable next to each other. After that you can have a CONTROLLED meeting, they will fight to settle who's more dominant, hair will be pulled and some fighting this is normal. What isn't normal is VERY aggressive fighting and when blood shows up, that's when you step in and separate them. One being not fixed might be a problem...he may be a lot more aggressive than the other. Even though they are both male one may hump the other, this is what the more dominant one does to show who's the boss.