It takes the same amount of horsepower to fly the same speed given the same gross weight and atmospheric conditions, whether AEO or OEI. There may be some small differences in the power required because of the installation of two engines, but not much. Specific range is calculated by dividing Vtrue by Fuel Flow. Since turbine engines are more fuel efficient at their higher power settings (in other words more hp per pound of fuel flow), two engines operating have to develope more horspower to get the best most efficient specific range. Of course drag increases by the square of airspeed and hp increases by the cube, so it is a trade off found by using flight test data to calculate the values. If the name of the game is to go as far as you can, fly OEI. Maybe not the safest thing to do.