With their individual quirks and personalities, ferrets are a lot like people. Just as people react differently to social situations, so do ferrets. It's important to remember this when introducing a new ferret to one that is accustomed to being the "only ferret." Ferrets are social animals, and in the wild, live in familial groups known as a "business." When a ferret is kept alone, you, the owner, become a part of that "business." Ferrets without another ferret to bond to will bond strongly with their caretaker. However, that is not to say that your lone ferret can't form a new bond with a new furry friend.
The trick is to take things slowly. DO NOT abruptly put the new ferret in your current ferret's
home, even if there's enough space. This will not go well, and you will have your work cut out for you to undo the damage before you can start over. Rather, keep the interactions as neutral and stress-free as possible. The new ferret will be under stress from being introduced to a new environment. Give him time to acclimate and feel safe and secure. Keep the two ferrets housed in separate cages at first. It's okay if they are near each other so they can see and smell each other. You can even swap pieces of bedding so they can get used to each other's scent.
Outside the cage, let their first interactions be short, monitored, and in a neutral area. Let them get used to the idea of each other. You definitely don't want your current ferret to feel like this new one has invaded his territory. A neutral territory could be an outside playpen or even a friend's house. That way, the ferrets are on equal ground, so to speak, and neither of them has "home field advantage."
You might try giving them treats or Furo-Vite to lick side-by-side, so they have an activity to focus on together. Then let the acclimating begin! Keep an eye on them to mitigate any real scuffles, but besides that, don't interfere. They will work it all out themselves in their own good time. Establishing dominance is completely normal behavior, so as long as no one's getting hurt, it's best to let them hash it out. If they keep fighting, separate them and aim for few, shorter interactions. Forcing the issue isn't going to make them accept each other faster; rather, it t will just stress them out. If one ferret is biting excessively, consider a deterrent like Time Out to make the weaker ferret taste bad and not as tempting to chew on.
Eventually, your two furballs will be weasel war dancing in step with each other. So when is it time for them to move in together? A good rule of thumb is to not cage the two ferrets in the same home until they are proactively curling up together while playing outside the cage. That's a pretty strong indicator that bonding is at hand!