I don't think there is enough data room to put 2D and 3D versions of the same movie on the same disc (plus special features). I could be wrong on this. It could be that it's more economically efficient to sell the the two versions on separate discs.
However, I did look it up for you and it looks like they are going to do a combo pack for the 3D release, which is essentially the movie with the 3D version, the bluray version, the DVD and the digital copy all in one set. So that's definitely your best bet are far as I am concerned. Lots of studios these days are moving towards putting the better special features on the 3D bluray as opposed to the regular and more studios are releasing these films in combo packs like this to deal with people being in various stages of adjustment. It's kinda a lousy system for regular people but I guess the idea is to get more people to buy 3D bluray.