Regardless of the wind, How much work the stoker puts out is up to the stoker. They have to pedal at the same cadence, but they don't have to put force to the pedals.
As for the wind affecting how hard it is to propel the bike forward, its going to have the same affect on the captain and the stoker because you're linked together.
The bike as a whole will be less affected by a headwind than a single bike, because you have double the horsepower (giver or take) but you don't have double the aerodynamic drag.
Think about your single bike analogy. If the second rider in the draft had his chain linked to the lead bike so that he had to help push the lead bike through the wind, they'd both be affected by the headwind. Well that's what a tandem is.