Yup there is some confusion about this. You are really just required to provide a 20a circuit for the "laundry". There is no restriction about how many receptacles can be on that circuit or what you can plug into as long as it is in the laundry.When I see a single 20 behind the washer location I understand someone has misunderstood where we are supposed to be going with this. They want you to be able to run the washer and the iron at the same time. Kids there used to be this thing called an iron ...
George Do you mean one of these? Just serving the laundry?
I am not sure what I would cite to require an additional circuit.
210.11(C)(2) Laundry Branch Circuits. In addition to the number of branch circuits required by other parts of this section, at least one additional 20-ampere branch circuit shall be provided to supply the laundry receptacle outlet(s) required by 210.52(F). This circuit shall have no other outlets.
Greg, I'm not sure of the amperage on the stacked unit but I was thinking it was 30a. Haven't seen one in a while. With that in mind, if it were 30a. the question becomes do you still need a 20a. circuit for the iron? or for some of us folks the Mangle. I think the Stacked unit is a 30a. 240v. single receptacle.