I'm not a fan of 36/40 spoke wheelsets for tandems.
If you ever crunch a wheel, it'll almost always be the rear and nobody is going to have a 40 hole rim in stock. If your front is also a 40, you can have your front rim laced onto your rear hub and substitute whatever front wheel you can find. A long time tandem team who are close friends of mine did exactly that in the middle of a week long tour.
I'd also worry more about the rim than the number of spokes.