Some where in my wanderings over the years, I read that the worst thing that someone can do is to learn each hand seperately and then put them together. The reason was that you are learning the piece three times. Once for the left, once for the right, and then putting both hands together. It is true that if you are having a problem with a particular passage, it is best to isolate what is giving you the problem and then work on the technique. However, it is the connection of both sides of your brain that allow you to play the piano with both hands, not always doing the same thing. Without establishing the connecting quickly, you have to struggle to get the nurons to cooperate. Have you ever learn to play a piece with both hands, and then practice a difficult passage with only the left and then find that the right had is twitching. It is like it wants to get in the game. Playing the piano is a two handed activity, and should always be done as such, except for the reasons stated above.