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.