No, you do not need 2 x CPU to do video rendering.
So what do I need?
Well...this depends on exactly what you are doing, what the rest of the system is, and what your budget is.
Making production quality, full length movies?
Making short video clips?
Capturing and rendering gameplay?
All would have different requirements.
And FYI, I can render video just fine on an almost 3 year old single i5.