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The two primary problems I see is that you can not ensure when the balance transfer to Citi will actually take place and this may mean that your AMEX is not paid in full by the due date and also if there is a balance transfer fee with Citi then even at the 0% interest rate you are still paying something more than what you would have anyway.
I guess I'm not really sure what you are hoping to accomplish with your plan. It seems as if you are making things unnecessarily complicated maybe in the quest for rewards points with AMEX?