But as you say, you don't go significantly faster by adding another piston engine, either. Aerodynamic resistance at high speed goes as v^2, so adding twice the power increases your speed at most 70%. In practice, it will be less due to increased drag from engines in the wings, more weight, etc.

I pulled a comparison from Wikipedia:

Cessna 310 (twin): 2x240hp, max speed 220mph, range 1000mi.

Cessna 182 (single): 230hp, max speed 201mph, range 930mi.

I'm sure the twin has more useful load, but it sure doesn't go much faster or farther.

Aerodynamic drag (the force required) goes as the square of the velocity.

Power required is force times velocity, so power required goes as the cube of velocity. Twice the power adds only 26% to speed.