Thanks, Sparky. You raised many important points that I didn't think seriously of. The idea of getting two degrees is from a person I know. She gets double PhD degrees in the same major, one in Hk and the other in the States. However, I don't the black box of how she did it. So I thought it might work.
Could you kindly share with me more about the structure of history department in the States and how is it related to the difficulty of getting double degrees, so that I can better evaluate my position? I appreciate very much your reply in advance!