On flat ground, I can't imagine that a tandem would be slower sans stoker, if the stoker is putting out any power at all, and is in even a slightly aero position.
The amount of aerodynamic drag added by the stoker isn't that much, and it wouldn't take much power output at all to overcome that.
Now going uphill you could be slower with a stoker if the stoker's w/kg resulted in a team w/kg that was lower than the captain's.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.