It takes X amount of horsepower to move Y amount of mass. Your horsepower moves your single bike mass at a certain speed. Can your stoker match your horsepower / speed if she rides a single bike.
i am guessing that she might be weaker than you are and in order to maintain the same speed that you ride on your single you have to attempt to produce more horsepower, thus the harder effort. You are attempting to move a 2Y mass with maybe 1.75X horsepower.
Just a thought.
Wayne