Max speed has very little to do with weight as long as you have sufficient room to accelerate. Additional drag would be small if you put one rider in front of the other and both inside of a fairing. The frontal area and shape could stay the same. This is the major source of drag. There would be small amounts of additional drag from the air running along a longer fairing, but doubling the power would more than make up for it.

However, the speed would nowhere near double. Power required to overcome drag force is related to velocity cubed. So if you double power, the velocity will increase by a factor of (2)^1/3 or the cube root of 2, 1.259.

So if they added another rider of equal power and managed to keep drag the same (it would actually increase a little) they'd go around 25% faster or 107 mph. You may run into other problems like stability and bumpiness first though.

As someone wrote above, weight doesn't matter that much, and since the most likely configuration would be one behind the other, the aerodynamics wouldn't suffer that much.