Mostly, yes, the one is for the front and one is for the rear, but not quite:
Left button: Blows air on the inside of the windshield from the dash vents under the windshield.
Right button: Turns on electric heating elements on the rear window
and electric heating elements in the side mirrors
and the electrical heating element under where the wipers sit on the front windshield.
View attachment 60787
Interestingly, the manual, above, implies some Siennas do not come with windshield de-icers, even though everything I've heard here points to the fact that they do.
P.S. Hey look, the manual says the outside mirror defrosters remove frost, dew, or raindrops from the mirrors! I hadn't read this, but I was already using it to do that! 🤣