This is a very common question that I receive.  I recently found a great article on the subject. The snippet below is just an intro, but you can click on the link at the bottom to see the full article.
 
Fighting for dominance is normal when introducing ferrets. The severity of which can range between almost no fighting to an all-out battle. It’s best to hope for the best, but “prepare” for the worst, which should make anything less than ferret to ferret warfare seem acceptable. It is important to be patient throughout the process. Many times things tend to settle down in 3-14 days however there have been instances where peace is not achieved for 3-5 months, or longer. In general, it appears the longer a ferret has been kept away from others, the longer it takes for them to adapt to the new space and time constraints. Aside from being rewarding, you can take comfort in your ferrets having a playmate readily available to them which can be a great experience for you and your fuzzies!  Read more...