The whole point of DVD2ONE, is that a single dual layer dvd movie nowadays requires two DVDs just for the movie.
DVD2ONE will reduce this to one, what you trying to do is totally different, as you are trying to fit two separate DVDs onto one.
At a guess I would say you are trying this with the special edition Lord of the Rings FOTR(or something similar) and I'm not sure it will be all that simple if possible.