YMMV, but a tandem is a great big averaging machine. I find riding the tandem to be much more physically demanding because of this. It is not that the stoker/captain is not pedalling as the over used joke goes but they are not pedalling quite as hard or with the same intensity as you are. And you can over exert yourself much easier on a tandem because it does not respond to the extra energy the same as a single (or a tandem without a stoker...). You put in extra effort and nothing much changes except you get more tired and more physical problems.