Apologies for another Titanic-related post this week, but this one has Mythbusters in it. James Cameron did an interview with IGN in which he talked about the conversion to 3D, yada yada yada... And then a minute before the end of the video, the interviewer asks Cameron if he's aware of the Reddit thread trying to debunk the whole "door couldn't hold both Jack and Rose" thing. Cameron argues that while sheer surface area may have allowed two people to lay on top, physics would not. It flipped when Jack tried to get on, you see. And the filmmaker says that Discovery Channel's Mythbusters will be tackling this mystery of buoyancy on an upcoming episode, and he would like to help them prove that he was right.
I feel like Cameron has actually run this experiment in his own personal laboratory numerous times, and that's why he's so sure of this. (Plus, science totally backs him up.) Though I still think that if Jeremy Sisto played Jack, he not only would have found a way for both him and Kate Winslet to get on that thing, he would have saved both of their lives with sex warmth.