Although I'm not interested in a side by side configuration, it could be that your tandem is 80 mpg plus, while your side by side is 350 MPGe.Force of wind exerted on a surface varies as the square of the velocity. This gets too complicated for me, but you can learn about it here: http://hpwizard.com/index.html
As a sailor, I know that a 12 mph breeze is wonderful sailing, but a 24 mph breeze is 4 times rougher, not twice as brisk. When you double the speed, you quadruple the force acting against you.
This is grossly simplified, but it's why at highway speeds aerodynamics are vital, more important than rolling resistance (tire friction) and all the other stuff. Sailors reduce the amount of sail area in order to stay upright and in control. Elio's reduce the square footage of the car to do the same thing.
At slow speeds, aerodynamic are unimportant, and if you want efficiency, you go for light weight. But over about 45 miles per hour aerodynamic drag is the villain, and as Paul says, half the width is have the wind resistance working against you.
So your side by side Elio is NEVER going to get good gas mileage at highway speeds, while your tandem is 80 mpg plus.
Hope this helps answer THAT question.