From what I´ve heard, a twin need only 2/3 of the power required for a single engine plane.
This because on a twin the propeller streams over the wings are more aerodynamically efficient

Here´s an old video showing my plane flying with 57 watts/lb (2x800W/28lb)
The performance is not great but good enough for scale flying

[link=http://www.youtube.com/watch?v=00oKuXjfxZI]Shrike [/link]