Depending on the situation, they basically mean the same thing, except that you would not use them the same way. For example, you could say either, "I have two reasons," or "My reasons are twofold." You cannot swap them. Twofold also means doubled, although I'm not sure that makes it any clearer. There is a similar word. "manifold," which is similar to "twofold" except that it means "many." For example, you could say, "I have many reasons," or "My reasons are manifold." You would not use twofold or manifold with physical objects. For example, you would not say, "My shoes are twofold," or "The flowers in my garden are manifold." These words are fairly formal and not generally used in casual conversation.

twofold does mean "to double" but in this context: "twofold" adj. - having more than one decidedly dissimilar aspects or qualities. http://www.thefreedictionary.com/twofold(scroll down the thesaurus part) Examples: My duties are twofold. I have to take care of the front desk and I have to park cars. I have a twofold reason for breaking up with you. For one, you cheated on me but also because I can't stand that you eat with your mouth open all the time. (This is just an example, not a personal experience :) ) To answer your question. "I have a twofold reason" or "I have two reasons" would essentially mean the same, but "twofold" makes it clearer that the "reasons" are unrelated to each other.

" twofold" means twice as great or as numerous, It is an adjective modified the noun (reason), twofold reason is singular. " two reasons" , two is a number more than one, two reasons is plural,