That’s more along the lines of what I was thinking. Even if you discount Houston’s double-dipping as POTUS after having served as President of the Republic of Texas (due to Texas having joined the Union, one could view this in the same light as former USSR bosses leading governments in former Soviet states, but kinda in reverse), simply having served as the chief executive of two different States is unique enough for my OP – in fact, Wikipedia confirms that Houston is completely unique in this regard.
Henri III of France having briefly been King of Poland is interesting too, I never knew that. All the Wikipedia article on him says in reference is: Three months after his coronation as King of Poland, upon the death of his brother Charles IX, Henri secretly left Poland and returned to France, where he was crowned King in 1575, at Rheims Cathedral. It seems the role of King of Poland was greatly circumscribed by the semi-republican Polish-Lithuanian Commonwealth, which he found unacceptable. I wonder what the Polish reaction was to his abandoning his throne?