There is almost a drum beat reference to the "One President" rule; however, no one says what it is and how it is enforced, for it is clear that Trump seems to think that he can yank the power of the presidency before he is sworn in.
The answer is The Logan Act (1 Stat. 613, 18 U.S.C. 95) brought into law in 1799 - that is not a typo. The Logan Act is:
Any citizen of the United States, wherever he may be, who, without authority of the United States, directly or indirectly commences or carries on any correspondence or intercourse with any foreign government or any officer or agent thereof, with intent to influence the measures or conduct of any foreign government or of any officer or agent thereof, in relation to any disputes or controversies with the United States, or to defeat the measures of the United States, shall be fined under this title or imprisoned not more than three years, or both.
In other words, the violation of the Act is a FELONY.
Regardless of the fact that Donald Trump is the President-Elect, he is still a private citizen until he is sworn into the Presidency - traditionally in January 20th.
So Mr. Trump, the "one president at a time rule" is not just another norm that you can ignore; a nicety that you can break because you just don give a damn about such things because you are TRUMP and can do whatever you want to whomever you want.
Obama and the Department of Justice do your job and enforce the law. Trump's continual intervention in international affairs is not just impolite. He did not just sh*t on the rug. Arrest him and get him convicted.
I am sure that folks next question is "Can someone who is serving time for a felony serve as President?" I don't know, but hopefully some Constitutional lawyer (hint hint Obama) could answer that question.